• Perks

• Quickload Interlink - increase your teammate dps too
  
• Thermal Venting - 25% more rate of fire during overclock
  
• Focus Fire - 10% more damage
  
• Suppressing Fire - slow your target by 25%
  
• Targets Acquired - the only recharge speed perk you need
  
• Mobility - synergize with Targets Acquired for more frequent CD reduction
  
• Disorienting Blast - knock down targets
  
• Mini-Grenades - knock down targets 3 times more frequent
  
• Down & Out - increased burst dps to targets after you knock them down

personally I use these for maximum shotgun burst dps for killing high-threat targets after knocking them down with grenade or activating overclock

• Tech CQW Training - 15% accuracy and reload
  
• Gunner CQW Mastery - 15% accuracy and rate of fire

or you could just use two 20% recharge speed perk for the grenade

• Gameplay Tips

Personal Tips

• Keep the stacks on your passive ability Stay On Target to the max as much as possible for more recharge speed.
  
• When there's more than 3 enemies nearby when you reload, throw a grenade first then reload
  
• When Drones or elites show up, wait till they get closer before using grenades and activating Overclock. Make sure they are in range for your shotgun to do optimal burst dmg.

Common sense tips

• Mutants(blue) gets too close for comfort warrants a grenade
  
• Use grenade to save grappled teammates
  
• Use Overclock when your teammate is about to get overwhelmed(forced to move away) by multiple mutants
  
• Memorize when elite targets(orange) spawn in each map and make sure your Overclock is ready when they appear
  
• Use grenade to probe the blind spots where Crawlers could be hiding. Or at least hold a grenade in your hand when you peek into those spots

• Perks

• Precision Rocket - turn the most broken class in the game into the most broken-op class in the game
  
• Quick & Dirty - synergize with Precision Rocket for triple skill cooldown efficiency
  
• Loud & Clear - 20% more damage and synergize with Quick & Dirty for more cooldown
  
• Fire & Forget - increase targets' damage taken by 25% with your rocket
  
• Bigger & Better - 40% more blast radius
  
• Batteries Included - 20% recharge speed
  
• Rapid Deployment - 20% recharge speed

Build No.1

This build leaning towards weapon performance, only works with a Recon in your team

• Down & Out - 20% more damage on knocked down targets
  
• Demo Heavy Training - 20% max ammo, 15% reload speed
  
• Demo Heavy Expertise - 20% explosive radius, 15% damage(the only perk in the game that directly increases weapon damage)
  
• 3 more slots to buff the rocket a bit more. I used three tier I perk cuz it gives tier I gives more stats value

Build No.2

This is my favorite build, the rocketeer

• All four Quick Recharge - ended up 100% recharge speed, halved cooldown.
  
• Tier II Enhanced Reach - more blase radius = more target hits = more cooldown, also you can worry less about nearby targets when you are trying to focus on a elite target

• Gameplay Tips

• Throw a rocket when there're 3 or more enemies
  
• Use rocket to probe blind spot for hidden Crawlers
  
• Dont use Blastwave too indulgently as its cooldown time is kinda long. Save it for crucial moments such as saving grappled teammates, knock down high-threat targets as someone about to get hurt badly etc.
  
• Recommend using rifles that can be equipped with anti-material break for knockdowns or Astra for cc(see weapon & attachment sections). Demo is not needing much weapon damage

• Perks
  
  

• Dynamic Delivery System - 10% turret rate of fire, 25% more damage to armor
  
• Parasocial Relationship - 20% more damage to targets your turret recently damaged
  
• Creative Pain Point Solutions - both you and turret deal 15% damage to slowed targets
  
• Particle Turret - much better than incinerator because its longer range makes Parasocial Relationship much easier to trigger, and it has slow so Creative Pain Point Solutions can trigger outside your charged coils. Also, no visual obstruction from all those flames. And an extra bonus for tryhards as incinerator turrets seem to have a bug of not registering its damage as your damage dealt in the mission performance in the end
  
• Compatibility Matrix(Charged Coils) - 10% extra damage for each active coil
  
• Maximized Retention - Stun targets that entered your coil by 2s. It says 33% chance but actually feels like 100%
  
• Down & Out - 20% more damage on targets stunned by Maximized Retention or your teammates
  
• Rapid Deployment(Charged Coils) - 20% recharge speed
  
• Quick Recharge II(Charged Coils) - 13% recharge speed. With 33% total recharge speed, the cooldown is 15s, exacly the same as it's duration meaning you can have at least one coil active 100% of the time

My personal build:

• Resonating Impact - Stumbled targets take increased damage from all sources for 6s
  
• Deep Leverage - When you or turrets shoots a target affected by Resonating Impact they get knocked down every 10s(15% chance but it happens in a second or two considering how frequently they get hit)
  
• Hyperlocal Logistics - 10% more turret rate of fire when you stand near it
  
• Force Multiplier I and II(Turret) - The particle turret deals considerable damage and definitely deserves a damage buff or two.

Other build:

• Scalable Machine Learning - up to 50% reload speed bonus as your turrets kills enemies
  
• Modular Integration - 25% accuracy and rate of fire on top of that 50% reload
  
• 3 slots for either turret damage or coil recharge speed

Machine learning vs Resonating Impact:
In my opinion Resonating Impact is much better:
- targets are easily stumbled, RI triggers quite frequently even on mutants or elites. While ML may not maintain maximum stack consistently if there's not enough enemies for your turret to kill.
- it provides frequent crowd control effect, cc is the the key in beating extreme and insane
- it increases damage dealt to target, increasing the damage of every bullet you shoot instead of allowing you to shoot more bullet, which means it saves ammo
- it increases damage dealt by all sources, your teammates too, not just you

• Gameplay Tips

• Throw coil to save grappled teammates(mostly for hidden crawlers. Doesnt work if the enemy is already being affected by a coil)
  
• Use coil to probe blind spot for Crawlers
  
• Try aim the coil at elite enemy's body, stick it on them
  
• Each active coil gets you 10% dmg, but only use 1 coil at a time and save the rest for emergencies. When your team are forced to move or when a new high-threat target appear, you'd be happy that you have more coils ready to use
  
• After combat, reclaim your turrets manually if it's not too far. Hit Q again may seem convenient but that 15s cooldown may screw you up if more enemies show up
  
• When encounter enemies while traveling, always throw coil first, then put down turret(if needed), never reverse order.

• Perks

• Suppression Station - slows enemies
  
• Peace of Mind - increase slow radius by 20%
  
• Enhanced Reach I II and III - increase radius further to a total of 58%
  
• Doctor's Orders - each member higher than 75% health gives you 5% dmg, total of 15%
  
• Strength In Numbers - each nearby teammate gives you 5% damage
  
• Support System - you heal 20% more, teammates near you gets 15% all ability recharge speed
  
• Readiness - increase reload 20%
  
• Charitable Glow - ability now increase your damage by 15%
  
• Overdose - run faster, last longer. Longer cooldown but it's an ability you dont use that much so it's a great trade off

If you are confident with your shooting skill go with pistol perks and use Type 78 Burst Pistol. This weapon has the damage comparable to that of the best shotgun if you score weak point hits up close, easily deal as much damage as a gunner or even demo.

• Technician HandgunTraining - 15% accuracy and weak point damage
  
• Phalanx Handgun Expertise - 15% rate of fire and accuracy
  
• Phalanx Handgun Mastery - 15% weak point damage and mag size

If you dont prefer damage then go with defense

• Bedside Manner - revive twice as fast and bleed out time 25% longer
  
• Rapid Recovery - give more hp when you revive them and bleed out time another 25% longer

For the other 4 slots it's really just personal preference. You get the luxury-level flexibility because tbh Doc is not a very useful class(very useful in specific missions) so his/her skill slots are not that valuable compare to other classes:

• You can use Anesthesiologist - 10% current hp as shield
  - Note that the shield will not recover if it's depleted by friendly fire(the last hit you took before shield depletion). In another word, it would only recharge if your HP(not shield) is damaged from an enemy attack
  
• Surgeon's Hands - immune to interruptions during interactions
  
• Or Enhanced Reach IV - EVEN BIGGER SLOW RADIUS
  
• Or Fitness Training - increase max hp and amount of heal you receive

• Gameplay Tips

• Trauma Station's purpose is not to heal but to slow enemies. Do not be discourage to deploy it for the sake of conserving healing charges.
  
• Even station with 0 healing charge left can still slow enemies
  
• This slow is stackable with other crowd control effect
  
• You shouldn't ignore it's healing completely neither. If combat is not too intense and your team can afford to care for details: let two team member heal using the station and the have the other member, preferable most injured one, avoid being healed by the station, and heal him/her with a med kit when hp drops low enough.
  
• Most important time to use Combat Stims is when team member(s) on the move with enemies around, such as moving towards mission objectives during combat; disengaging from enemies when being overwhelmed etc; leading away enemies and running back to revive downed teammates etc.

• 2-3: The level where Doc actually shines for their ability to heal. And you can tank synthetic with Anesthesiologist perk. Just take cover quickly to regain shield once it's depleted.
  
• DLC-3: Another level in dire need of extra heal. Also If your team is very experienced with that mission and well-coordinated, you can potentially use Doc's Combat Stim to rush through the first maze
  
• Any mission with double hp card.

All mission above work better with Anesthesiologist equipped.

• Perks

• Red is Dead - most op skill in the game. Your team gets 10% increased damage all the time(yes, all the time. See weapon sections)
  
• Got Your Back - instant reload when you killed an enemy that's targeting teammate
  
• Back in the Fight - on top of reloading you both also gets 10% skill cooldown reduction
  
• Down Range - increase far distance damage by 20%. You're gonna be in the back most of the time
  
• Down & Out - the universal 20% damage increase perk. You cant stun/knock down enemies, but your teammates still can
  
• Distracting Howl - slows target marked by PUPS
  
• Blood Hound - lowers marked enemy's defense. It's strategically the best variation of PUPS.
  - Since your team is gonna equip laser sight guns, the only purpose of default version PUPS is to make targets visible for better visual acquisition;
  - Guard Dog has decent crowd control but it's point-defense not AOE, it's not guaranteed to slow all enemies unlike other cc such as tech or doc(but best for fighting synth);
  - Best friend only provides more dps but no cc which is a deadly disadvantage compare to other variations;
  - Blood Hound not only slows the enemies but reducing enemy defense, achieving cc and dps increase at the same time, and most of all, it has unlimited duration and no cooldown so you can free up skill perks for other skills. Yes it only weakens one target but why do you need to weaken every other enemies that takes less than 1s to kill and possess no threat to you what so ever, and when a 2nd high-threat target shows up you ended up in cooldown and unable to use it again?

Despite being the best support class and almost a must-have for extreme and insane, it has the most flexible skill perks due to it's abilities doesnt need any blue perks to buff them.
Personally I'm going all out on CQW perks. CQW Shotguns dps are only second by heavy weapons, and such investment yield insane damage:

• Tech CQW Training - 15% reload and accuracy
  
• Gunner CQW Expertise - 15% rate of fire and accuracy
  
• Tech CQW Expertise - 15% rate of fire and stability
  
• Recon CQW Expertise - 15% weak point damage
  
• Lancer CQW Expertise - 10% weak point damage
  
• Adrenaline Rush - Drone can actually heal quite a lot(10 base heal per enemy)
  
• Quick Recharge I - 7% recharge
  
• Extended Duration I - 7% duration

If you are confident with teammate's damage output and dont feel like playing so aggressively, I recommend
• Bedside Manner - revive twice as fast
  
• Surgeon's Hands - cant be interrupted during interactions
  
• Perks that gives Support Drone duration and recharge speed

• Gameplay Tips

Personal tips

• When there're multiple mutants(blue) at close range, do not try to mark them one by one, just shoot them all. The time it takes to mark one mutant is enough to shoot off 1/4 of it's hp, which means you ended up killing them slower if you tried marking them
  
• Between a spitter and an elite(orange), mark the spitter and kill it asap then mark the elite. Unless the elite is chasing you and your teammate is not attacking the spitter
  
• During horde fights, stay in the back in order for Got Your Back(instant reload weapons) to trigger properly

Common sense tips

• Dont mark runners, only mutants and elites
  
• Drop ammo as soon as it's ready when the team stays in position or passing through an area that you are going to visit again.

• Perks

First I apologize for not being very experience with phalanx, neither with its build nor gameplay, and I havent beaten much extreme/insane mission with it. I'm just gonna write down what I have. Consider checking other people's guide if you are really into phalanx builds

• Shield Bash - shield melee attack stuns target, use it on the one that grapples
  
• Take A Stand - each nearby enemies increase your damage by 3%
  
• Aftershock - leaves a small slowing field after using skill
  
• Concussion Pulse - larger radius, knocks down targets
  
• Batteries Included - 20% recharge
  
• Quick Charge II - 13% recharge
  
• Hazard Seals - 25% acid resistance
  
• Down & Out - 20% more damage to knocked down targets
  
• Tech CQW Training - 15% reload and accuracy
  
• Recon CQW Mastery - 15% weakpoint dmg and range
  
• Tech CQW Mastery - 15% rate of fire and stability
  
• Gunner CQW Mastery - 15% rate of fire and accuracy
  
• Lancer CQW Expertise - 10% weakpoint dmg

(I focus on shotgun perk to do more dmg to high threat targets while on the front)

• Gameplay Tips

• Learn to identify which animation/sound effect of the enemies that indicates grapple attack and slam it with shield melee or trigger shockwave before it grapples
  
• Enemies gain period of immunity to stun after shield melee, so only use the slam when you want to interrupt grapple
  
• Shockwave have a very long starting animation, it can be tricky to time if you want to interrupt grapple, especially on Drones with it's high movement speed.
  
• Keep a distance from the rest of your teammate(though in most cases it's your teammate needs to keep a distance from you) so the enemies lock on to you.
  ESPECIALLY WHEN FIGHTING PATHOGENS, if the exploding bugs failed to lock on to you and moved past you towards your teammate then exploded, both you and your teammate will take damage.
  
• Your shield blocks friendly fire so feel free to use the U1A2 grenade launcher to blast the ground in front of you while holding a shield to kill surrounding enemies.

• Perks

• Rapid Assessment(Particle lance) - max stack passive upon use
  
• Residual Charge - leaves electric pool for 4s upon kill
  
• Omniscience - increase massive passive stack from 4 to 6 and slows down stack decrease
  
• Tenacious - 5% damage increase, ability lasts longer
  
• Relentless Advance - passive stack doesnt decrease during ability
  
• Rapid Deployment(Particle lance) - 20% recharge
  
• Batteries Included(Particle lance) - 20% recharge
  
• Quick Charge II(Particle lance) - a total of 53% recharge speed
  
• Force Multiplier I II and III(Particle lance) - a total of 38% damage increase
  
• Demo Heavy Training - 20% ammo, 15% reload
  
• Demo Heavy Expertise - 15% explosive radius, 10% damage, the only perk that directly increase weapon damage

The reason I do not use particle pulse is that it does not bring out the strength in lancer. Particle pulse is all about crowd control, it does much smaller damage. You cannot fire weapons nor reload while casting this skill and the casting animation is a bit long, meaning you actually lose a portion of dps every time you use particle pulse. Enemies recovers quickly due to gaining immunity period after being stunned, and the slow effect only last for 5 second(3 second if you account for skill casting time). It just makes lancer a weak crowd control and low skill dps, jack of all trade with a discount.
Compare to damage build default lance, which takes out 3/4 hp of Burster, 1/2 of crawler, 1/3 hp of spitter on insane, with only 20s cooldown, makes him quite a day saver and damage dealer, and an absolute carnage when fighting against synthetics(one shot regular synthetic with 50% skill dmg card).

Because lancer's particle lance is very perk dependent, and needs the heavy weapon perk for ammo and damage, leaving lancer little to no flexibility in the skill perks
If you have to take off something then I'd say maybe the Residual Charge.

• Gameplay Tips

• Use lance skill on spitters, or group of mutants(blue), or a train of runners. Only use it on elites(orange) if no more mutants spawns. Make sure you wait for the right moment for lining up shots against multiple enemies.
  
• When using Onslaught to kill elites, if they are close enough, use the shotgun instead of heavy.
  
• When a random mutants spawn during travel, wait for a few second before using the lance, because they tend to spawn in group of 4, especially on insane, and you want to make sure they all finished spawning and make your lance hit most or all of them.
  
• Lance casting time is quite long and you cannot cancel it once initiated, if enemies are launching attack especially like spitter or synth sniper, you risk of getting hit is huge. Make sure you use it at the right time

First there's another guide detailing damage performance ranking of every weapon in the game if you are interested:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3006393203

Weapons with laser sight have a consistent 10% extra damage when a recon is in the team.
Now to summarize best weapons to use:

When a Recon in the team:

• L36 Halberd - the best overall performance rifle
  
• M41A2 Pulse - same non crit dmg ad Halberd, but 10% less crit dmg. However it has a larger mag which would synergize better with recon's mag refill skill.
  
• Type 88 Heavy - higher dmg and crit dmg than both, but it's accuracy and recoil is bad
  
• EVI-87 Plasma - as high dmg/crit dmg as type 88, but it's dmg drop quickly over long range

Without a Recon:

• Kramer - highest damage without recon buff
  
• 4C2 Astra - almost as high damage as Kramer but with much better accuracy. It also has an op crowd control build, see attachment section

Recon doesnt matter here:

• L33 Pike - tailor-made anti-synth weapon. It even have the best damage performance, as good as CQW - if only if you can headshot every time and able to shoot right away without wasting time to aim the scope

When a Recon in the team:

• Type 21 Tactical - the best damage
  
• EDS-93 Plasma - Almost as good as Type 21, but it can sacrifice some dps to it's ability to knock down target with Anti-Material Barrel

Without a Recon:

• M37A3 Pump - highest crit dmg without recon, highest dmg per shot among shotguns. Despite being single reload it's overall reload speed is not terrible
  
• M51 Breach - highest non crit dmg without recon
  
• Type 76 Auto - also very decent damage, but during overlock it could be a bit tricky to handle

Recon doesnt matter here:

• X1 Fireball - best AOE in CQW weapons, better than the Incinerator. Only use if you do not intent on dps with shotguns.

Recon doesnt matter here.

• Type 95 Combat - despite not having laser sight it still has excellent performance due to it's high mag size, accuracy, low recoil and long range. Easy to get frequent headshots
  
• Type 78 Burst - its crit dmg is as high as the best shotguns and out damage every other pistols by a mile. Perform similarly as a CQW
  
• U1A2 Grenade - the only AOE weapon in pistol slot. If you have a CQW as dps weapon then it'd be a good idea to pair with this launcher for crowd control.

Recon doesnt matter here.

• L59 Minigun - highest dps, 2nd highest crit dps, and able to equip Anti-Material barrel for knockdown, but only good at close range because its bad accuracy makes it had to maintain headshots
  
• P.649 Hel - dmg second by minigun but highest crit dmg, best against synth and elite(orange) xenos as well as spitters. Great at all range single target dps
  
• 2B1 Varja - 3nd place dmg amoung all heavy weapons, and it's the only heavy weapon capable of applying additional slow effect(with 4 star) and makes it god tier crowd control dps weapon. Being an AOE means it can clears runners but it also means risk of friendly fire. However it has the smallest blast radius which means it's the safest AOE weapon to use.

Instead of listing them one by one(except the special ones), I'll explain stats priorities you'll be looking for.

• Rate of Fire - get it right away
  
• Weak point damage - get it right away
  
• Accuracy - for all CQW, and for rifles that has lower accuracy than 77(Pulse rifle's accuracy, which I find is the minimum acceptable accuracy for long range consistent headshot)
  
• Stability - only concerned when gunner's overclock is involved, for high rate of fire weapons only
  
• Max ammo - if you are not playing with a recon, you are gonna need all max ammo you can find. Even when you are playing with a recon, extra max ammo cant hurt.
  
• Reload - good to have
  
• Mag size - less important than reload
  
• Stumble chance - can be ignored. All guns stumbles if you shoot them enough times
  
• Effective Range - not helpful as only increase the minimum damage at long range, and do not increase the optimal range for maximum dmg. Also it does not increase the range before shotguns reach 0 dmg
  
• Handling - is a joke. It increase the speed you switch weapons and enter ADS.
  
• Aim assist - for psychological reassurance. I don't feel any actual difference even with that 50% aim assist card equipped.

These are for general use:
(ones you want to put on every weapon)

• Laser Sight(Medium Optics) - Reveals target synergize with Recon's Read is Dead skill, and it has weak point dmg and accuracy, all desired stats, the no.1 god tier attachment in this game.
  
  
  
• Miller Twist Rifling(Barrel) - slows targets by 50% indefinitely. It's the thing that makes AOE and heavy weapon shine aside from their high damage. It has 20% chance to trigger and last 10s but you usually trigger in 1 or 2 seconds and it's duration refreshes all the time. It's not good for M12 Rocket or other rocket but seriously nobody use those on Insane unless you want to literally one shot the whole team or ended up 5k+ friendly fire in performance.
  
  
  
• Anti-Material Break(Large Muzzle) - knocks down targets every 10s. Has 25% chance to trigger but again it happens in just a second.
  
  
  
• Assault Break(Medium Muzzle) - its stats are just broken.
  
  
  
• Tactical Scope(Large Optics) - counterparts to the laser sight for snipers
  
  
  These are for specific weapons:
  
  
  
• Duel Rail Booster(Barrel) - for P.649 Hel. This weapon is the highest single target dps in the game and many high-threat targets have armor. This barrel increase it's dmg by a mile.
  
  
  
  
  
• C36B5 Karachi Overwatch and Inhibitor Rounds - for 4C2 Astra. Being the only rifle able to equip both these attachment, allows it to slow target by 60% half the time and 20% all the time, making it a very OP non-heavy non-skill crowd control. Especially useful for Doc, a class that's not dps-oriented.
  They also can be used on Type 95 Pistol and some smgs but the pistol slot have grenade for AOE, Type 78 burst for close range dps, while rifles have neither. Not to mention Type 95's amazing accuracy recoil rof and mag size makes it a very decent long range dps weapons. Overall, using these attachment on pistols sacrifice more functionality than using them on rifles.

When handling elites:
• When an elite(orange) got too close while other enemies are still coming in numbers, NEVER have all 3 team member firing on the elite at the same time(unless someone is using AOE as dps) as it would allow other enemies to pass through your chock point, break your defense formation and put the whole team at risk. What you should do is,
  1. identify which team member it's locked on,
  2. whoever gets locked on starts retreating and lead the elite away from the team while the rest 2 members keep holding the line(having an elite walking past by right next to you may seems terrifying but keep calm, as long as you keep focus on other enemies and the player it locked on keeps attacking and attracting it, the elite wont bother you one bit)
  help your teammate kill the elite after everything else is dead
  
  however if your team just got overwhelming firepower and tons of crowd control effect, cryo traps, etc. then great, vaporize it where it stands.
  
  
• If someone with a shotgun is at a closer range ready to kill a drone or elite, the best thing you can do is to move out of their way, dont risk friendly fire, only support with crowd control skills and more importantly help cleaning up enemies around the shotgunner allowing him/her to burst dps that target with no interruption.
  
• Ideally, let whoever the target locked on be the short-range dps

Spitters:
• When a spitter is spotted, kill it asap. If you can't, then move out of it's line of sight asap. Never stay put giving it another opportunity to shoot at you.
  
• Spitters are THE most threatening units in extreme and insane. They always attack you when your focus is locked on other enemies, then take out a third or half your hp with one spit.
  
• Camp where spitter doesnt have a clear shot on you, force them to come closer where you can see them and easily kill them.

• When your teammate hits you, chances are IT'S YOUR FAULT, NOT THEIRS(unless they are idiots carefreely using rockets or other team-wiping AOEs). Pay attention to your positioning in narrow spaces. Keep in mind that when your teammate are aiming down sight, they are unlikely to notice you, like you literally dont show up on their screens, so try NOT to:
  - move/roll past their line of fire right in front of them
  - stay too close behind their back
  - dodge backward/sideways when your teammates are right behind/next to you
  (You may justify saying you have to do these maneuver to dodge attack, but you shouldn't gotten yourself into these positions in the first pace. Be mindful where you choose to stand and shoot)
  Dont make it a challenge for teammates to not hit you by moving recklessly when people are firing.
  
• As for yourself, in tight spaces minimize movement while firing to prevent unexpected teammate from poping up on your scope

Prioritize using long range weapons like rifles to conserve short range weapon ammo, save the short range weapons like shotguns or type 78 for mutants or enemies coming too close. This way you dont have to put any Total Ammo perk or attachments on short range weapons so you can go all out on weakpoint dmg and RoF for maximum burst dps.

• Door holding is having the rest of the team stay by a door while someone go to the mission objective and trigger the horde, then run back to the door and use the chock point to fight the horde.
  
• If no one is holding the door, some doors will shut for good. Even when it's opened by enemy reinforcement later on, an invisible barrier will still prevent you from going back.
  
• Most objectives wont trigger until all members stepped inside the room for a certain distance. You'd need to get deep enough into the room to trigger the object while stay close enough to the door to keep it opened. Move slowly, one step at a time.
  I lost count how many times people who think they are smart but failed to realize this, I signaled them to step inside to trigger objectives, then they left, thinking I was telling them to not hide behind a door :)
  
• Objectives telling you to Rally or Gather cannot door hold. It needs all of you inside, you cant stop the door from locking up.

PS: Door holding is technically a glitch that's not part of the intended gameplay. But it's something that almost everyone use(especially if you play matching) and people have developed strategies, habits and builds revolving around this technique, which I assume is the reason why devs never patched it.
If you do not wish to play with exploits or glitch and be a chad, it's definitely great but best to find someone who's willing do the same i.e. not matching. If you are playing matching, it's best to learn to cope and play along with door holding to avoid complications.

• Your ability to score weak point shots is crucial. Keep practicing.
  
  
• Learn these info:
  - where to camp
  - which doors to hold
  - crawler locations
  - when/where elite enemies spawn
  - when/where drones spawn
  - when/where hordes that spawn as soon as you walked to a certain place
  - how to read elite enemies' attack moves to dodge at the right time, so you can solo them/lead them away with no problem
  
• If you are not completely familiar with those information, you must have someone in your team who does. Knowledge is power.
  
  
• Dont waste med kits. Amount of meds drastically decrease on higher difficulties. Depends on your awareness of your own skill(and your network lag if you're the guest player), only use med kits when your hp fall below a threshold of risking of being downed. However, this threshold should be higher before an intense battle where you probably wont even get the chance to heal.(Personally I heal myself only when below 300 health on insane difficulty, and when below 500 before an intense battle)

Cards that can be useful on extreme and insane. I may have missed a few cards here.

• Half CD card with 100% rocket recharge Demo can stun any xeno enemies indefinitely(5 rocket, 1 shockwave, repeat, no cooldown gap). Nothing can touch you other than spitters(or your teammates). And you can shoot them while spawning rockets and shockwaves, you are basically a walking war machine.
  
• Demoralized weakens Insane difficulty elite enemies almost to the strength of Intense difficulty

• Synthetic assist card also on this tier, sorry for not having the image
  
• Ammo drop and Half Speed both effectively double your total ammo and mag size. Though half speed might take some getting used to. But once you do, you'll find it's lower recoil and slower crosshair spread very appealing especially during overclock
  
• High Threat Env provides 50% dmg for both weapon and ability damage, outperforming any other damage card by a ton. It just takes skills to use. Wont hurt being a glass cannon if you dont get hit. Just dont use it on levels with pathogens or synth(guns and exploding bugs, both hard to avoid taking damage).
  
• Double HP is just a no brainer and works very good with doc(temporary hp shield), and work even better when fighting synth or pathogens.

• 50% ability dmg, lancer > demo > gunner > tech
  
• 35% weapon dmg, gunner > recon > tech > lancer > demo
  
• Heavy synth card may be risky to use because the spawn is random but it works wonder in some places, especially pathogen levels. In DLC-3 if it spawns in the boss room then it'll draw away all the attention of runner spawns.

Those are better than no cards.

• Dodge allows you to dodge consecutively which can be very helpful in certain rooms. On Insane if you get hit you lose like 300 health, but that much health allows you to dodge 12 times.
  
• Acid immune cannot block damage from the direct hit of spitter projectiles, but it can block dmg from the acid pool formed below you after the hit. Work best with Phalanx.

or cc for short, it means any effect that stuns, slows, knocks down or stumbles enemy targets