I'm going to continue to add achievements to this guide that made me scratch my head.

"Who guards the Guard?"
Achievement description: "Win a solo game on impossible difficulty against Astra Militarum on a huge map."

"Mission Improbable"
Achievement description: "Win a solo game on impossible difficulty by finishing the story quests."


The solution to this is actually really simple:
Tweak game settings, complete in-game quests, and win!

Step 1:

In the game lobby, while on the Basic Settings tab. Set the landmass to the lowest, "Very Low".

The idea is to create a narrow path between you and the Imperial Guard. As well as to limit their expansion potential.

Step 2:

Open the "Advanced" settings tab. Set the "Wild Life Density" and "Wire Weed Density" to very high.

This is to bog down the Imperial advance and their narrow path to victory. Your base!

Step 1 & 2 should give you something like this below:



Step 3:

Try to complete your quests before they reach your base.


Simple right. Use the game settings to buy you the time you need and some breathing space for those quests where you need to visit locations on the map.

I got this achievement before the new "Mission Improbable" achievement, but technically you should get both. Because as far as I know you just need to win, not seek and destroy.

Achievement description: "Win a solo game without killing any units that are not headquarters."

This has multiple ways, but there's one very easy way I know of:

Plan A:
Step 1:
Pick the Space Marines faction against an opposing Space Marines faction.

Step 2:
When picking your city location, try to find a location rich in Influence.

Step 3:
Try to boost the influence to a rate per turn where you can Scan and Orbital strike each turn the enemy city.

Step 4:
Do a battleship style grid search with your scan to try and locate the enemy city. Start early. Most maps are mirrored in a way. So starting your Orbital Scan grid search by guessing where the enemy city probably would be is a good start. Try to keep a 1-2 map tile units spacing between the Orbital Scans, as to use the least amount of Influence while maximising the area searched. Think of it as creating spots on a black canvas. Instead of a clean sweep of every single map tile.

Step 5:
Start your orbital siege of the city until you win.

If all goes well you should be able to destroy the opposing Space Marine city by the time they reach your base.
You could try and reach the enemy city with flyers, but I end up getting hit by tons Fortress of Redemptions.

Plan B:
Pick Guard and go against Space Marines (only has one city).

Step 1:
While in the game lobby, set the research speed to the highest and land mass to smallest.
This allows you to amass a small air raid operation near their base before starting the assault.
Step 2:
Rush for Guard fighter (does more direct damage) and heavy bomber (tank).
Step 3:
Create small air raid force and time your assault to overwhelm their city defense.

If the space marine Fortress of Redemption ability is giving you trouble then try decreasing the special resource amounts.


GADver 's approach:

Step 1:
Take Aeldari (standart difficulty, if not - no achievement)
Opponent - Astra Militarum(very easy)
Smallest map
Step 2:
Defend the gate they attack first (closest to enemy) - you can shoot em away just don't kill the whole unit.
Step 3:
Go for vehicle path - you have 5-6 you can destroy their base.

"That was my way to victory =)" - GADver

Rhana Dandra:
"Win a solo game on impossible difficulty as Craftworld Aeldari on a huge map against a team of 2 AI Chaos Space Marines."

This is my low landmass choke point approach:

This is all about very precise timing and shaving turns off of quests before the enemy shows up roughly around turn 64.

Key things to note:
* Build heroic units ahead of time as not to waste unnecessary turns.
* Try to get to the Scorpion tank eventually, but not as the only priority.
* Guardians are good units for most of the fighting, although squishy.
* Anti-armour units are key. As well as units that act as damage spunges.
* Make use of Autarch unit accuracy boost and the Farseer Skyrunner morale boost and damage boost versus monster units.
* Research speed set at highest. (In my experience they always showed up at around turn 60 regardless of research speed.)
* Lowest landmass and largest size map.
* Try to get a forward base near the area where the quest units keeps showing up. As you will need to be able to heal fast. And maybe hold of the enemy for a few extra turns before they reach your main base.
* Set the map Webway Gate setting to maximum (found in Advanced Settings). As this would allow you to get a good estimate of how far along the AI is on their way towards your base. As they destroy the Webway Gates with their approach.

Research should always be ahead of time. Build these units even before the quest shows up:
- Autarch.
- Rangers.
- Farseer Skyrunner.
- Avatar.

When a quest ends I've found completing a research node immediately starts the next quest.

The most important thing is a large map with lowest land mass possible and the fastest research speed. As I found the enemy roughly shows up with the same units around the same time regardless of the research speed you choose. And the small land mass restricts mass ground unit movement.
Also, you need at least 2 Scorpion tanks when the enemy shows up as I've found this unit can deal and take a lot of damage. So you are able to keep the enemy at bay and focus down a unit every now and then with the proper timing and support of heroic unit abilities.

The map settings I chose:
Set most sight blockers (Ruins, forests, etc anything that units can't see past.) to very low, while setting wire weed and rivers high.
This is to quickly spot the quest enemies, while also bogging them down and allowing you to pick superior positioning and keep them contained.

You want to be building units which counter the quests units. So go for that.

Focus more on Guardians and Fire Dragons than Banshees and Rangers. 1-2 Rangers not bad, but they only scale well if you use the Autarch accuracy boost and Farseer Skyrunner damage ability as a combo. They can then output insane damage for a turn against heavy units.

The Eldar artillery hits multiple units. So it's best used to shoot first to thin out squishies, like basic infantry, while the Rangers pick off the remaining few.

Final stages:
The enemy is going to show up eventually with roughly 20 flyers (Helldrakes) around turn 64.
You need at least 1-2 Scorpion tanks built or being built by then. As well as already being busy with the very last quest mission.
You want to hunker down in your main base to receive the defensive buffs. You do not want to get caught in the open. While waiting for the quests units to attack. Some will be destroyed by the enemy. So just try to survive during the last quest.
Use the Autarch accuracy boosts and Skyrunner morale boosts and damage ability.
I tried Warwalkers but they were to squishy.

-----------------------------------------------------------------------------------------------------------------------

CarloArmato's large landmass approach:

"But I wholeheartedly disagree on the "very low landmass" setting. With high or very high landmass, you have multiple pros:
* there are more "nature" units to fight: Chaos AI will chase this "neutral" bigger buffer, while you can hold your position mid to late game
* you have more outpost to conquer (which also escalates with autarch lv6 skill)

On my winning attempt, I spawned in the south east corner, I triggered the last quest on turn 50~ and spotted the enemy AI at turn 60~ and left the alone to chase neutrals. By turn 70~, I won with just one unit lost against chaos (a quest relic left to hold an actual relic with damage buff)
My best attempts were with high landmass (with the 2nd being the winning one).

High river density, low forest and ruins tips still applies, but IMHO medium neutral unit density and low to medium wireweed are slightly better.

I've built a 2nd city with the only purpose of research and resource (mostly food and energy), so I was able to spam crimson hunters (I almost killed the avatar of Khaine in one turn with ~5 of them) and got 2 or 3 Scorpions by the end. Actually, during the last quest I had such an overkill army around the spawn areas that I had to skip turns because there were multiple turns with no spawns at all. IMHO Crimson Hunters are a better choice early on because they are very fast and deals a ton of damage when 2 tiles away, but you need a ton of energy to produce and for the upkeep. My tactic was to keep them between my spotters and blast whatever spawned right away."

Dodge in da window's approach:

Achievement description: "Turn all the tiles on the map into bedrock."

A thank to "Dodge in da window" for this one:

Choose your faction: Tyranids.
Opponent: Choose Chaos, but doesn't matter.
A.I.: very easy
Basic map settings: Smallest map with land mass very low.

Eliminate the enemy asap, warriors and hive tyrant should be enough.
After winning, select the option to continue fighting, and kill all the neutrals units.
Start spamming Malanthropes, and reclaim the units which has no purpose anymore (like fighting).

Meemai's approach:

""Rock around the clock"
This one can be achieved by starting the game without any opponents".

Achievement description: "Win a solo game by only creating Haruspex units."

Thanks to "Dodge in da window" for this one.

Rush for Brood Haunt and Haruspex reserch (tier 3 and 4).
You can only train Haruspex so you have only 1 city. (Founding new cities not an option.)
Some research boosting structures doesn't hurt, go for all the upgrades that improves the Haruspex unit. Like armour, extra attacks/abilities, and damage, etc.
Try to apply the Iron Curtain approach by squeezing your enemy to death with an ever enclosing hungry Haru stampede.

Achievement description:
"Start a match in a team with another Tau player."

Choose Tau as faction and add a Tau AI teammate.
Start the match.
Done.

At first I thought you needed a buddy for multiplayer, but it seems they don't have to be human.

"Tau Can Play At That Game"
Achievement description: "As Tau, kill a unit with melee."

"Dark Messiah"
Achievement description: "Kill a unit with an Ethereal."

As simple as it sounds.
Just finish of a low hp enemy/neutral unit with an Ethereal.
You can get both of these achievements early in the game at once.

Quite simple as it's mostly a little luck with the map layout and just holding out.

Pick the Necrons as they don't require you to found a second city. The Necron Lord hero unit also allows you to choose a population cap increase as a leveling ability. This saves you space on not having to build population cap buildings. I had 4 Necrons Lords near the end, 2 Crypteks, and two Destroyer Lords.

Make use of the Necron teleportation back to base to save your units.

AI: 2.
Difficulty: Impossible.
World Size: Tiny.
Land Mass: High.
Game Pace: Very fast.

I set all the "advanced settings" to very high, to see what would happen. This gave a lot of cover for infantry.

Your biggest challenge is fighting towards the middle of the map, where the artifact spawns which you must capture to start the last quest.

Most of the enemy units will be dealt with by the AI, so just teleport back to base if you are caught off guard by the spawning quest units. So make sure to have a large influence pool.

I'm adding this to the list for those wondering on how to achieve this.

You need to play as Tau and use their ability "Convert Auxiliary" on a Kroot Hound unit. Then just use the Kroot hounds to kill another Kroot hound.

Description: "Execute a coward."

This is probably an easy one. As it's the Commissar of the Astra Militarum that executes cowards. So just use their ability "Summary Execution" on a infantry unit.

Description: "Kill a C'tan."

If I'm correct C'tan units will show up when you do the Necron quests.

Description: "Use rapid rise."

This is a Necron ability. You can use it while producing units to almost instantly build them at a large influence cost. The ability is found next to the production queue of the city build view.

Description: "Reclaim a unit."

This is a Tyranids achievement. The ability is found within the city menu. Similar to the Necrons rapid rise, except you must bring a unity close to the city and then use this ability on them.

You don't have to be Eldar to get this achievement.

If you want it fast, then just play against an Eldar opponent and set the Webways to the lowest value in settings.

Then just wait while drinking tea.

Achievement description:
"As Adeptus Mechanicus, win a solo game on impossible difficulty by producing units only from Sub-Cloisters."


Settings:

Difficulty: impossible.
World size: tiny.
Land mass: very low.
Game pace: very slow.

Choose Eldar as the opponent and it's important to set the Webway Gate density to very low.

Advanced:
Wildlife density: very high.
Webway Gate density very low.
Wire Weed density: very high.

(The rest of the settings I think doesn't matter much.)

Orkoid Fungus density: very low.
Imperial Ruins density: Very high.
Forest density: very high.
River density: very low.
Region size: very small.
Region density: very low.
Arctic region density: very low.
(The rest was all medium.)

Rush Sub-Cloister and use the tech-priests to capture Kastelan Robots. I got 5 in my play through.

By turn 107 the Eldar was able to field their first tank, but only after I was already busy focusing down their base.

If you get lucky and move fast enough, then you can destroy the Eldar's possible expansion Webway Gates before the Eldar gets a chance to activate any of them. This was what happened in my case.

Pop out 2 tech-priests engineers, followed by a tech-priest Manipulus, and a tech-priest Dominus.
The tech-priest Dominus can be skipped though. A third tech-priest engineer can't hurt.

Level up the tech-priest Manipulus' healing ability. Move the tech-priests with your units to keep up the healing and momentum.

A 2nd base can't hurt, but it might not be needed.

If all goes well, it might feel like easy mode. Good luck.

I think there's two ways about achieving this.
1. Refer to the "Adeptus Monotonous" entry in this guide (just don't build a second city).
2. Play with Necrons and win by doing the quests, while only building one base.

Approach 2 was as follows:

Take a tiny map with 2 AI on impossible difficulty.
The land mass set to medium and also choose fast tech.
Then set all the other settings to max.
I chose random A.I. (if you wish).
(I ended up against the Imperial Guard and Space Marines.)
Focus on quests while not building any new cities.

The goal is to have the A.I. tie each other up, while you just try to do mini pushes for quests towards quests locations.

Description:
"Kill a hero with a Ratling."

I got it killing a Tyranid Prime. What a hunt!

Explosive Delivery:

"As Drukhari, deal 20 damage in one attack with a unit with Raider Assault."

Steps:
Use a Raider to transport Kabalite Warriors within range of a target enemy vehicle or structure. Disembark the Kabalite Warriors within one tile of the target and use their Haywire Grenade ability on the enemy vehicle.

Notes:
This was achieved with a level 4 Kabalite Warriors unit.
"Raider Assault" has been researched.
The target enemy unit was I used it on the Space Marines Whirlwind, which was a neutral unit during Chapter 2's invasion.