Hello there, Director.

I have completed Conglomerate 451 Story Mode on Hard difficulty, and I would like to share my experience with you, in hope that it'll help you master and enjoy this beautiful game.

There are probably many ways to win battles and complete the game, but in this guide I'll be mostly sharing what worked for me, while trying to explain why it worked.

Feel free to tell about your own strategies and experiences in the comments. :)

The key to your success is creating a squad of three agents that can handle any fight they find themselves in.

First off, you need to pick the clone backgrounds (roles) that work well together. I recommend using the following roles:

• Infiltrator
  
• Soldier
  
• Drifter

They are the first three roles available, so you will be able to start using and upgrading them from the first week.

Note that you will need at least 2 copies of every role, in case one of the copies has to get sent to healing, or gets sent to a special assignment that appear sometimes alongside usual missions.

Why these roles and not others?

• Soldier - He is kind of mandatory because of his Shield Regeneration ability, which will allow you to restore the shields of your party mid-fight. The ability to regenerate your shields will allow you to survive in extended battles, especially against corporate bosses or 5+ opponents.
  
• Infiltrator - Has the highest Attack, Critical Hit Damage and Range out of all the roles. Her Critical Chance is only slightly behind Binary, and her Base Damage is second only to Soldier and Juicer. All of this combined makes Infiltrator the best DPS role, allowing her to quickly dispatch almost any enemy.
  
• Drifter - Drifter gets in the squad mostly thanks to his Death Mark ability that applies the Mark effect, which weakens enemy defense and synergises with Infiltrator's Death Shot ability. He can also apply Radiation effect with Swift Strike, which in turn synergises with Soldier's Single Shot. A nice bonus is that Drifter also has the best Hacking and Hardware stats in the squad (second only to Binary and Techie), which should give you a slight boost when hacking doors/enemies or when Extracting SPUs.

However, it should be noted that Infiltrator herself can apply both Mark and Radiation, so you can basically replace Drifter with a second Infiltrator. The reason I personally stick with Drifter are his two additional abilities:

• Improved Weapon - It can further increase Infiltrator's damage, allowing her to basically one-shot everything.
  
• Battlefield Medicine - Just like Soldier can restore Shields, it's also nice to have someone who can restore Health. However, the situations when your shields are down and you are taking health damage should be very rare, and if it came to that - you've probably already lost the fight.

Now that we know the three roles we need, we also need to know the abilities with which to clone our agents.

First off - forget about area of effect attacks (Blast and Conflagrate). Your goal should be to quickly destroy targets one by one, thus reducing incoming damage and increasing your chances of survival, so you need attack abilities that deal the most damage to a single target.

Infiltrator

• Death Shot. -15% base damage, but deals +35% damage to marked targets. When the target is marked, this is Infiltrator's most devastating attack skill.
  
• Torment. -15% base damage, but applies radiation for Soldier to exploit.
  
• Spark. Removes Stun and Mark from the whole squad and adds +5% vision to everyone. This skill can be useful in some rare specific situations, but you can probably replace it with Accurate Shot (which doesn't have base damage penalty) for the also rare situations when you fail to apply Mark to your enemies.
  
• Accurate Aim. Buffs self with vision +10%, critical chance +10%, base damage +12%. If you can spare a turn to use this skill, it really turns Inflitrator into a killing machine.

Soldier

• Single Shot. +25% damage to enemies affected by radiation. Good synergy with Infiltrator and Drifter, who can both apply radiation.
  
• Stunning Shot. Allows to temporarily stun an enemy, thus reducing incoming damage.
  
• Shield Regeneration. Regenerates 50% of a squadmate's shield. This is an essential ability or keeping your squad alive.
  
• Barrier. Increases the defence of the whole squad - good for reducing incoming damage, so that you can regenerate shields less often.

Drifter

• Death Mark. Applies the mark effect (which increases crit chance by 10%) to an enemy and reduces its defence by 20, although the ability itself doesn't do any damage. But the Infiltrator sure will. ;)
  
• Swift Strike. Applies radiation effect and does decent damage.
  
• Battlefield Medicine. Restore 5 HP of any squad member. Useful if someone managed to break through the shields.
  
• Improved Weapon. Increases squadmate's base damage by 25% - Infiltrator's going to be happy.

Now that you have your clones with specific abilities, you need to know how to use them in battle. Below I'll describe specific movesets that you can use to maximize your damage.

Always try to attack a single enemy until he collapses or dies, then switch to the next enemy. This way you will

The order your agents take turns depends on their Initiative stat, but most commonly the order will be: Drifter (70-80 Initiative), then Infiltrator (50-60 Initiative), then Soldier (20-30 Initiative).

There are two general types of opening movesets: long (when you ambust the enemy) and short (when you get ambushed or there is no ambush).

Short moveset. This moveset is useful when you're facing weak enemies that you want to dispatch quickly, or if you want to try and quickly kill someone in a situation when you don't have much time.

• 1st turn
  
• Drifter: Death Mark. You should try to mark an enemy as soon as possible to maximize Infiltrator's (and also Soldier's) damage.
  
• Infiltrator: Death Shot if mark was successful (deals the most damage), Torment if there's no mark (sets up soldier).
  
• Soldier: Single Shot if Infiltrator applied Radiation with Torment or if enemy is low on HP and can be finished. Otherwise use Stunning Shot or Barrier to try and reduce incoming damage.
  
• Repeat.

Long moveset. This moveset is good either if you ambushed your enemy and have extra time to set up, or if you're facing a strong or numerous enemy and must raise your damage as much as possible.

• 1st turn
  
• Drifter: Death Mark. As with the short moveset, you should start with applying mark to maximize damage to the target.
  
• Infiltrator: Accurate Aim. Significantly buffs herself.
  
• Soldier: Barrier if you're expecting a lot of incoming damage, otherwise you can do some extra damage with Single Shot.
  
• 2nd turn
  
• Drifter: If the mark was successful - use Improved Weapon on Infiltrator, which buffs her even further. Otherwise, try to apply mark with Death Mark again.
  
• Infiltrator: Death Shot. At this point, Infiltrator is buffed with Accurate Aim, Improved Weapon, Mark on the target and Death Shot's own bonus vs marked. That sums up to a huge base damage bonus and an almost guaranteed critical hit, and should destroy or severely damage most enemies.
  
• Soldier: Single Shot to finish the enemy if Infiltrator didn't, or Stunning Shot to reduce incoming damage.
  
• While Infiltrator's buffs last (3 turns), keep applying mark with Drifter's Death Mark and keep using Infiltrator's Death Shot to destroy marked emeies. Then either reapply buffs again according to the long moveset, or switch to the short moveset to finish the remaining enemies.

If your Infiltrator's Death Shot is not yet upgraded, use Torment instead and then finish the enemy off with Soldier's Single Shot. You can also use this combo if Drifter's mark failed to apply, making Infiltrator's Death Shot inferior to Torment (since Torment applies Radiation, and Death Shot doesn't apply anything).

If all the buffs and marks are applied and your Drifter doesn't know what to do, use his Swift Strike to deal some damage and also hopefully to apply Radiation.

Closely watch your squad's shields. If anyone's shield falls below 50-60%, then your Soldier needs to immediately use Shield Regeneration, regardless of the current move rotation. It's also a good idea to try and keep Soldier's Barrier up, so that you can worry less about incoming damage and concentrate on outgoing damage. :)

If someone on your squad is receiving too much damage due to being marked, or your Soldier got stunned and you urgently need to restore shields, remember about Infiltrator's Spark that can cleanse mark and stun effects from the whole squad.

If the enemy managed to breach your shields and damage HP, but your Soldier managed to intervene and restore the shields, remember to use Drifter's Battlefield Medicine to restore HP in case the shield gets breached again.

If your agent's shields and HP are both down to zero and he's about to die, my advice is to immediately leave the mission and save your agent, so that you can avoid losing all the resourced you spent on upgrading him. You're unlikely to bring him back to full health after this happens, and successfully finishing the mission (and the fight) with just 2 agents is rather unlikely.

Other than using your agents' abilities, another option you have during battles is hacking.

Right now, however, there are very few situations where you'll find hacking viable. Hacking uses battery, which is a very limited resource, and it also ends the agent's turn, which could instead be used for an ability that would kill or stun your enemy.

There are 3 hacking plugins that could, in theory, assist you in some battles:

• Beacon - reduce enemy damage.
  
• GlintBase - reduce enemy vision.
  
• Slick - apply mark to an enemy.

All their effects last for 3 turns.

The first two plugins can help you greatly reduce incoming damage in situations where it's overwhelming, while the third can help kill an enemy faster. You should normally use the first two and the third plugin on different enemies, because the marked enemy is supposed to die soon afterwards, so his damage or vision shouldn't need reduction.

In my opinion, reducing damage is always better than reducing vision, but theoretically there could be a fight where the boss deals such huge damage that it should only be avoided and not reduced. However, I haven't found such a fight, and reducing damage always worked for me (in the 1 or 2 boss fights where it was needed).

When choosing the level of plugins, pick the one that will allow you agents to use the hack 2 times. So, for example, if the agent has 60 battery available, pick plugins that will use 30 battery each.

In practice, the only time you will likely need hacking is during corporate boss battles, because the boss can inflict huge damage to your agents, if it's not reduced or avoided. It may also be useful in fights against 5+ enemies, which can also deal a lot of combined damage that should be quickly reduced. In any other battle, your Soldier should be able to outheal incoming damage to your shields, so no hacking will be required.

So, imagine you're in a situation where you're taking a lot of damage and your Soldier can't outheal it. Who should start hacking?

• Drifter. You should try using hacking after he's already applied mark, or use a second hack to apply mark during the same turn. Other than marking, Drifter's abilities aren't essential, so his turn can easily be spent on hacking.
  
• Infiltrator. If your Drifter's battery has run out and you Soldier is super busy trying to keep your team alive by regenerating shields, then you'll have to spend your Infiltrator's turn on hacking. And it's a shame, because she could deal huge damage during that turn otherwise.
  
• Soldier. If both your Drifter and Infiltrator have depleted their batteries, or if you find an opening where their shields are up and you have already activated the Barrier, use hacking with Soldier.

If you've managed to master the battles, then all that's left is to gather enough resources and defeat the corporate bosses.

Try to pick missions that will give you the most Tech, but don't go on missions of 2nd or 3nd difficulty until you've defeated at least one corporate boss - at this point, those missions won't feel difficult to you. You should also know that there's not much difference bettwen the 2nd and 3rd difficulty, so you can try doing the 3rd difficulty missions even when the game tells you that your squad is too weak.

You need to reduce a corporation's influence to 30% or below before you can try and kill their boss. For me, the first boss battle was the toughest (I lost 2 squads there before I managed to score a win), while the other boss battles I completed on the first try without much difficulty.

Even after you kill all corporate bosses and reduce all corporations' influence to zero, the game is not yet over - it will only end after 75 weeks.

There are three possible endings that depend on how you react to special events and assignments: one when you side only with the government, one when you side only with the rogue agency, and one when you try to side with both of them.

The first step of every mission is the preparation in the city. I recommend to not skip this part, because you can get extra SPUs and resources by looting defeated enemies, as well as activate multiple beneficial bonuses for the mission.

There are a few useful spots in the city that you'd be wise to visit.

Selling plugins is a good source of Tech. You can safely sell all plugins except those that reduce damage (Beacon), reduce vision (GlintBase) or apply mark (Slick) - all other plugins and their respective hacks are useless.

And you really shouldn't buy any plugins - just wait until they drop naturally.

Like with plugins - don't buy any SPUs here, just wait for them to drop during missions.

You can safely sell any basic SPU that grant useless stats, like Survival Instinct, Max HP or Rresistance to Debuff Effects.

Vision and Critical Hit Chance are okay, but your agents can only raise these stats up to 100, so at some point they'll become useless too.

Best SPU stats that you should try to keep are Base Damage, Attack, Defense and Max Shield.

The only times you may really need drugs is for fights against corporate bosses. But, since the credits are abundant, you might as well keep a few drugs handy, just in case.

The useful drugs are:

• Spirit - Defense +10
  
• Morbid - Base Damage +25%
  
• Burnout - Attack +20

Each of them only costs 500 credits and adds 5 intoxication, and their effects last for 3 turns. I suggest buying 3-4 each and then feeding them to your Infiltrator if things get tough.

You might also consider buying Smut (1500 credits, 15 intoxication) - If you suffer Critical Hit, next turn your damage will be increased by 50%. This can really make your Infiltrator's damage skyrocket, at the cost of high intoxication.

Crisps (2000 credits, 10 intoxication) removes all debuffs and may also help your Infiltrator quickly regain some temporary lost Vision/Initiative/Defense/Attack/etc. Keep in mind though, that your Soldier's special ability is also removing all debuffs when he performs a critical hit (which will affect either Infiltrator or Drifter, 50/50 chance).

Here you can find some really interesting things, although they'll cost you some Tech.

The things you want to look for, if they are available:

• Oracle - Increased chance to master a skill at the end of the mission. Mastered skills are both useful and difficult to acquire, so you should really pick this software every time it's available.
  
• Tempest - Increases collected Techs at the end of the mission. Techs will be your most limited resource, so you'll want to get this one as much as possible, especially for missions with high Tech rewards.
  
• Level Up - A chance to double rank progression at the end of the mission. If the first two options are unavailable, or you want to quickly climbs your squad through the ranks, this is a good software to get.
  
• Graveyard - Chance to inflict mark at the start of combat. If you're nervous about the upcoming mission, this software might make fights a little easier. If it succesfully applies mark to your enemies, then your Drifter can skip his Death Mark move and use Improved Weapon on your Infiltrator on the first turn, allowing to reach higher damage one turn faster. This works best for missions with multiple fights and I wouldn't recommend picking this for a corporate boss fight.
  
• Snowflake - Chance to inflict radiation at the start of combat. Same as above, but instead it helps your Soldier do additional damage with Single Shot at the start of the fights.

If you see any of these things - just buy its highest available tier.

This also contains some interesting things. They only cost credits, and since credits are abundant, you should really always pick something.

• Overload facility - more credits and Tech, but no map. Having no map can be annoying, but extra Tech is always useful, so consider taking this option as much as possible. I also recommend this for corporate boss fights, since they don't really need a map. Synergises well with the Tempest illegal software.
  
• Extra rank - +50% rank progression, but stronger enemies. Personally I didn't feel like enemies get significantly stronger, so this is a good and safe option for progressing your squad ranks faster. Synergises well with the Level Up illegal software.
  
• Proximity sensor jammer - ambush all enemies. This is a good option if you want to make your mission and your fights easier. If you're really nervous about your corporate boss fight, you can pick this to give you an extra turn.
  
• Unlock all doors. This is a very nice quality of life option if you didn't pick any of the above options. Also good for the endgame when you want your missions to end as fast as possible.
  
• Overload REMs - get more SPUs from enemies but they ambush you all the time. I've honestly never been in a situation when I lack SPUs, but if you find yourself in that situation - pick this.

Once you've completed all of the city preparation stuff, find the elevator and get down to the mission.

During the mission, the hardest part is the fights, and you're already prepared for that. ;)

Other than that - try to use your scanner in every room to detect possible sources of SPU. And also try to use the battery recharge only if all your agents really need that battery - it only works once. Press all the switches and collect all the access cards - they open doors.

If you've decided to complete the mission without a map to get extra Tech, I suggest trying to always move in one direction, i.e. always turn right (move clockwise) or always turn left (move counterclockwise) - that way you'll visit everything and never get lost. You may, however, stumble upon a locked door - in that case either try changing your movement direction (e.g. from clockwise to counterclockwise), or be ready to move through the whole level twice, or just hack the door (if you have the battery).

After you finish your mission and return to the agency, there are some things you should do.

If your mission went sideways and you didn't immediately leave like I recommended, one or more of your agents may have died. The first thing you'll want to do is clone new agents, just like the ones you've just lost.

If your Tech allows, try to always pick the mastered version of abilities (if available) and then make at least 1 or 2 upgrades to them. It's more effective to upgrade every ability 2 times than to upgrade one abiliy 4 times.

Upgrading your shield and weapon is generally more effective than upgrading your abilities, because upgrading your shield will directly and significantly increase your survivability, and upgrading your weapon will significantly increase your base damage and crit chance (which aren't very important for your Drifter though).

If one or more of your agents can be promoted, try to promote them as soon as your return from the mission. It's best to do this before you send your agents to healing, because you can't promote them while they're being healed, and you'll have to wait until the end of the next mission for their healing to complete.

For recommendations on which promotions to pick, read the guide further.

You should closely watch your agents traumas (happen due to high pain) and disorders (happen due to high toxicity). Once a trauma or a disorder become permanent - your agent is bad goods, and you'd be best to replace them. This is especially true for the "fear" traumas that reduce damage by 35% against certain opponents.

So, as soon as your agents gets a disorder or a trauma - send them to healing immediately and hope this disorder or trauma goes away. If they didn't get healed on the first try - send them to healing again next week, until you succeed.

While you still have Tech available, spend it on research to improve your agents.

In the Cloning Research tree, all you need are Dichiussulfyde, Octapsabdaene and Fribscer DNA mutations, which are located one after another in the middle branch. Basically you don't need to research anything else in this tree.

In the Healing Tech tree, try to unlock the 4 discounts first, which will decreased the resource cost for further research by 40%. Then try to get 3 tanks each (regeneration and detox) in case your shole squad gets traumas/disorders, and lastly increase the tank effectiveness so that there's a better chance for traumas/disorders to go away.

In the Training Tech tree, you should also try to unlock the discounts first, but here it's trickier. First unlock level 1 upgrades for defense and attack (and their respective discounts), then unlock 2 more attack upgrades and the upper discount, then unlock 2 more defense upgrades and the lower discount, and then the last two level 4 upgrades. Prioritize attack first because it will help you kill enemies faster, which in turn will reduce incoming damage. The best defense is a good offense. ;)

Military Tech tree is the same as Training Tech in terms of getting discounts, although here you might want to prioritize shields over weapons. Having more shields will help you to survive against corporate bosses and 5+ enemies, and it also improves Soldier's Shield Regeneration, since it regenerates a % of shields (so it regenerates more with each use if you have more total shields).

Cyber Tech requires spending the most Tech, so even getting all discounts here is going to be tough. Realistically though, the only thing you need to research here is the first Prosthetic Leg, which will significantly increase your Stun and Radiation resistances. Everything else you can live (and win) without.

If you've done all of the above and you still have spare Tech - spend it on upgrading your agents and their gear.

Prioritize upgrading weapons and shields, then install implants, then upgrade their abilities, and lastly upgrade their cypher deck.

Decoding Echoes in Archive doesn't really do much for the gameplay, it just tells you the story and the lore of the world you're playing in. So I advise to wait until the endgame, when you'll have an abundance of extra Tech.

My advice is to use 2500-3000 Tech on decoding each Echo (60-70% success chance). This way you'll have less failures, so you should actually be spending less Tech overall.

Someone smarter than me can probably make a math formula to calculate the best option, but my gut tells me 2500-3000 is right. :)

Here's a reminder about how spent Tech translates into Echo decoding chances:
3500 Tech - 80% success chance
3000 Tech - 70% success chance
2500 Tech - 60% success chance
2000 Tech - 50% success chance
1500 Tech - 40% success chance
1000 Tech - 30% success chance
500 Tech - 20% success chance

Each attempt (both failed and successful) will decrease the integrity of the Echo, so you only have 10 tries for each.

To be successful in battle, it's important for your agents to have the right stats. These stats can be modified with mutations, promotions, implants and SPUs.

One of the first things you'll want to increase is Vision - nothing is quite as frustrating as missing your shots. Vision can only got up to 100%, so, once you reach that value, you should stop investing in it. This stat is important for all your 3 agents.

Like Vision, Critical Hit Chance can also only go up to 100%, and you'll want to increase it as much as possible (but, again, don't try to increase it past 100%), especially for your Infiltrator. All agents have a special ability that activates on a critical hit, so critical hits are useful even for abilities that don't do much damage (like Death Mark).

Critical Hit Damage is a very important stat for your Infiltrator, but, sadly, it's randomly assigned when you create a clone (from a range of 2.5-3.5) and cannot be modified afterwards. You can try creating multiple Infiltrator clones, picking the one that has a Critical Damage of 3+, and then discarding the others.

Defense, while it's measured in %, it will be very difficult to raise up to 100%. It's still a very important stat to invest in (more important than Max Shield), because it reduces incoming damage for each hit. This stat will be useful for all your 3 agents - they will all be taking damage.

Base Damage can, and should, be raised as much as possible, particularly for Infiltrator and, to a lesser degree, for Soldier. There's no limit to how much it can be raised. For Drifter this stat isn't important.

Attack also doesn't have a limit, but 80-100 is a good target value for reducing enemy defense. This stat is also important for Infiltrator and sometimes Soldier, but not very useful for Drifter.

Max Shield is important for your agents' survival, so you'll want to increase that with upgraded shields as soon as possible. SPUs can be better spent on other stats though.

Initiative is sometimes good to take a turn before the enemy (allowing you to kill or stun him before he can deal damage), but it's not very important. You can try to increase Soldier's Initiative to make his turn happen closer to your Infiltrator and Drifter.

Max HP and Survival Instinct are useless stats that you should avoid increasing. They are useless because, if you're doing everything right, the enemies shouldn't break through your agents' shields, much less bring their HP to zero.

Stun and EMP resistances are the most important - you don't want your agents to skip turns because of being stunned. Agents start with a ~100% EMP resistance, but the Stun resistance you'll need to increase (from ~40% to 90+%). EMP resistance gets decreased when you install implants, so you'll need to compensate by installing SPUs that increase EMP resistance in your new implant (as with stun resistance, try to keep it above 90%).

It's important to remember that the Debuff resistance doesn't encompass what is included in other resistances, so it won't save you from mark or radiation effects. It will, however, save you from effects that reduce your stats (vision, base damage, crit chance, initiative, etc). This can be important for Infiltrator in order to not lose her DPS, but generally you can just rely on Soldier's special ability to clear debuffs.

Radiation and Shock resistances will save you from the respective damage over time effects. These effects don't do any significant damage though, so you can usually just endure through them.

Having Mark resistance is nice in theory, but your agents start with 0% resistance, and raising it to a meaningful level (like 40-50%) is not viable. So your best best is ignoring this resistance and endure through marks, or just cleanse them with your Infiltrator's Spark ability.

Disorder and Trauma resistances start at around 100% and you won't be able to significantly modify them. These resistances will decrease as your Intoxication and Pain levels increase. Mostly you can just forget about these resistances and just heal your agents when they get a trauma or a disorder, although you will still have to finish the current mission with the suffered trauma/disorder.

As I mentioned in the research section of the guide, there are only three mutations you will need:

• Dichiussulfyde - Base Damage +10%. This is the first mutation you'll be able to unlock, so it can be useful for your early agents.
  
• Octapsabdaene - Vision +10%. This is the mutation you'll want to pick for your Drifter, beause increasing his Base Damage wouldn't do him much good.
  
• Fribscer - Base Damage +10% and Critical Hit Chance +15. This is obviously an impoved version of the Dichiussulfyde mutation, so you'll want to use it for your Infiltrator and Soldier as soon as it's available.

It's worth noting that your first cloned agents will be without mutations, so, for maximum effectiveness, you will at some point need to discard them and re-clone them with the newly unlocked mutations.

There are three general promotion paths for all roles, and then there are specialized promotions for each role when you reach certain ranks - Private I (2nd rank), Corporal (6th rank), Colonel (11th rank). I've yet to reach the Colonel rank in my playthrough, so I don't know what its perks are.

General promotions
Offence Path. Increases Base Damage by 3% and Initiative by +5. This whole guide we've been trying to maximize Infiltrator's damage, so this is natrurally what Infiltrator should pick every time. This path is also good for Soldier after his stun and EMP resistances are good. Keep in mind though, that at some point Infiltrator's initiative will surpass Drifter's, so their turn order will change.

Support Path. Increases resistances to Stun, EMP, Radiation and Shock by +5. This can help your Soldier and Drifter increase their Stun and EMP resistances to above 90% (though don't forger to install the DFL 335 implant for +40% stun resistance). After both your Stun and your EMP resistances are over 90%. you should stop picking Support path. Since your Infiltrator won't be picking this path, she'll have to increase her resistances through SPUs.

Defence Path. Increases Max HP and Max Shield by 3%. This is what your Drifter should pick after his resistances are in a good place, in order to further increase his survivability.

Specialized promotions
Infiltrator
Private I - Tactician. +25% crit chance against enemies with full HP. The hardest part is taking down enemy shields, so they'll have full HP most of the time.
Corporal - Vanguard. A critical hit gives +30% crit chance for Soldier or Drifter. Since their special abilities activate upon a critical hit, this should increase their rate of activation (and also increase overall outgoing damage, obviously).

Soldier
Private I - Striker. A critical hit gives Soldier +10 defense. It's a nice additional damage reduction so that soldier has to regenerate his own shields less often.
Corporal - Tactician. A critical hit regenerates Soldier's shield by 25%. As with the previous perk, this will allow Soldier to be more sustainable, allowing him to focus more on doing damage or on healing other agents instead of himself.

Drifter
Private I - Tactician. If the battery is higher than 75%, Damage suffered is reduced by 50%. This is a slightly worse but more readily available version of a crucial SPU effect (described below). I'm not sure if they stack, but it will help your Drifters before you can get your hands on those SPUs anyway.
Corporal - Striker. A critical hit gives Drifter +50 Initiative. This helps Drifter catch up with an Infiltrator who went deep into Offence Path, and also allows him to have a turn before most enemies (so he can, for example, finish off someone weak before they get a shot).

The most important implant you will need is Prosthetic Leg DFL 335 (it's the first leg implant that you can research) - it will give you the much needed +40% resistance to stun. It's recommended to install this leg for all your agents.

Another useful implant, and one that is initially available to you, is Arm Prosthesis ASA 252 - it will increase your agent's base damage by 15% and defense by 10. This will come in handy (see what I did there?) for your Infiltrator.

Further down the Cyber Research tree you will find Head Implant MDH 634, which gives an additional +30% damage bonus against marked enemies. However it comes with a significant downside of -40 EMP resistance, so you'll need to regain that resistance back through implant's SPUs. Also keep in mind that you'll need a lot of Tech and time to research this implant, so you'll probably only get it in endgame - don't try to spend all your starting Tech just to unlock this implant at the start, it's not worth it.

Similarly, you can research a Exoskeleton STB 782 implant, which gives a +30% damage bonus against enemies affected by radiation - can be useful for your Soldier. It also requires less research to unlock than MDH 634.

A honorable mention goes to the Arm Prosthesis MDA 612 implant, which increases your agent's damage by 50% after suffering a critical hit (basically a permanent effect of the Smut drug). In fights where you Infiltrator suffers critical hits every turns, this implant can help her gain more DPS than the aforementioned ASA 252 implant. However, researching this implant will require A LOT of Tech, so you'll probably be better off skipping it and just using the initially available ASA 252 instead.

Don't bother with any other implants. I recommend only installing Leg DFL 335 to all agents and Arm ASA 252 to your Infiltrator - that will have your needs covered for the whole game.

SPUs give the widest variety of stat modifications, and they can be installed in your weapons, shields and even implants. Gen 1 and 2 only modify your stats, while Gen 3 to 5 can have some very beneficial additional effects. So, naturally, you should try installing the higher gens whenever possible.

Keep in mind though that you can't remove or otherwise reuse installed SPUs - they are installed permanently and they're gone if you replace them with another SPU or if your agent dies. For your first agents (the ones without mutations and the ones you'll likely discard) you might want to use cheaper SPUs that you won't mind losing.

For the guide on which stats to increase and why - see the important stats section. Below I will only mention the useful special effects that you can find on Gen 3-5 SPUs.

The most important effect you should look for is "If your battery is higher than x%, the damage suffered is reduced by 50%". So this basically cuts all incoming damage in half, making your battles significantly easier. I strongly suggest installing SPUs with this effect for all your agents before you attempt your first corporate boss battle or harder missions. Keep in mind though that you shouldn't spend all your battery (like on hacking enemies) for this effect to remain active.

"Damage Bonus vs Marked/Radiation" is a good pick for you Infiltrator and Soldier respectively, to further increase their lethality.

"Damage Bonus vs Human/Cyborg/Android" is also a good choice, but obviously won't work in all situations.

"If the agent is not the first to move in a combat round, their Resistances are inreased by x%". Your Soldier will never move first in combat, so it's a nice permanent resistance bonus for him.

"If the squad is attacked by more than one enemy, increases Defense by x%". Similarly, most of your fights will be against more than one enemy, so it's a nice permanent Defense bonus for any agent.