Aliens: Dark Descent

Not a Guide, since this game is very dynamic. But Tips on my Playthrough.
[Highly recommend playing a regular playthrough before you start a Nightmare run]

1. Only go for Main Objectives, get resources when its necessary. Try to complete missions in a single deployment if possible, so infestation level doesn't rise (furthest I got was Lv3). Try to keep Alien Aggression on Easy or Undetected, if detected weld a door to stop aggression progression.

2. Recon should be your main deployable class, on EVERY mission. Precision Shot generates no noise with the Silencer Upgrade, which lets you take out non-boss units quietly even when spotted, requires Sniper Rifle. Infrared Googles Upgrade lets you highlight enemies, sometimes through closed doors/walls using the flashlight.

3. Medic/SGT should be your next classes. Reasons below.
Class Attributes/Upgrades
Medic: Health points +1 all Marines/Decrease stress when healed/Healing interaction -50%
SGT: +1 command point/command points recharge faster/bravery total +15

4. Tecker and Gunner weren't needed. Teckers drone is useless, and the Smart-Ass [hack encrypted doors] attribute can be obtained by any class. Theres no huge damage difference with the Gunner.

5. Retribution Skill [LB+RB] generates Command Points faster, letting you spam any skill.
Avenger common attribute generates x% more retribution points for Retribution Skill.
SGTs Command Backpack increases Command Points by 1 [stacks] and generates Command Points 10 sec faster [stacks].
Team Spirit common attribute increases Command Points by 1 for every 2 Marines with Team Spirit.

Playthrough
-move fast, kill patrolling Xeno with recon
-rest when stress is Lv2 before leaving deployment to avoid trauma
-motion sensor your path every so on, can be used in the future to lure enemies away
-lure enemies to the APC vehicle with motion sensor or if chased by a boss
-flamethrower a close door to prevent Xeno from coming in, works better if welded
-run on big open areas, walk on close corridors, use RT to slow time when spotted/Recon clean-up
-queens die faster with Grenade/RPG launcher

Common Attributes Needed
-Team Spirit
-Avenger [can Lv up]

Common Attributes for fast deployment
-Unbreakable Will
-Mind of Steel

Common Attributes for damage
-Dead Eye [can Lv up]
-Sharpshooter [can Lv up]

Common Attributes that makes Marine more reliable
-Smart-ass
-Looter [can Lv up]
-Hardened [can Lv up]
-Redemption [only if personality trait cure applies to character]
-fast [can Lv up]
-Bold [can Lv up]
-Tough [can Lv up]
-Resourceful

As with a lot of difficulty achievements, the game is too ever-changing to give a 1-1 guide on exactly how to play. That being said, some general tips that have been helping me progress are:

1) Practice the stealth approach on a lower difficulty
• Difficulty in Dark Descent controls damage, stress, and health. It does not change objectives, routes, nor how detection or pathing works. If you can manage to do most of a mission stealthily, it is effectively the same experience on nightmare as it is on easy.

2) Become familiar with Marine Classes
• There’s a lot of variety in Dark Descent, but a good chunk of it is objectively worse when going for a best run. The most useful class order I’ve found is Recon > Sergeant > Medic > Gunner > Tecker.
• Recon is your prime asset for stealth, as their Precision Shot one of very few ways to eliminate foes silently. The sniper is always a one hit kill on normal enemies. A must have on every mission.
• The Sergeant helps you refuel Command Points quickly, and the Reprimand ability keeps your squad from becoming overly stressed. This is useful both in short and in the long term, as stress is the hidden killer in this game. Bring one whenever possible, preferably equipped with the flamethrower so that your bravest marine is the one responsible for area denial.
• Medic’s role is obvious. Bonus health, healing, and in some rare cases, can reanimate Marines / Civilians who have fallen into a coma.
• Gunner is useful for defense, and only defense. They’re great in a fight, but you want to avoid every fight you possibly can. Still, they’re worth bringing along and leveling up so they can be ready for the fights that you cannot avoid.
• Tecker is a very odd role that needs further dissecting. It’s made almost completely unnecessary due to the fact that your normal marines can also learn to hack terminal doors via the Smartass Common Perk.
They have two unique traits, one of which is their little drone that can be sent ahead to scout, make noise, weld / unweld a door, or give minor battle assistance during late game. There’s certainly *some* usefulness here, such as luring an enemy to an ambush spot without having to waste a command point on making noise, but it’s so limited that it’s not really worth locking you out of the other classes.
Their second trait is being able to upgrade sentry guns once they hit a certain level. This is an exceptional ability, but it’s so situational that most players will never even use the ability because, by the time you can make use of this trait, you’ve enough supplies and experience that you should be able to deal with the threats with or without the upgraded sentry.

3) Master your special weapons.
• The shotgun is useful during the beginning / midgame, but it falls off quickly.
• The flamer is a must bring weapon, but you only need one. It is technically a silenced weapon, making it ideal to destroy eggs and for area denial to ensure nothing sneaks up on you. Enemies that are not alerted to your presence will not walk through fire, nor be alerted by it.
Additionally, a flamer can be used on a welded doorway, as the damage of the flames leaks through the doorframe and an alien bashing against it will take tick damage and quickly die before they can get the door down.
• Mines are another must bring, and it’s the only weapon I’d suggest having two marines carry. Mines are immensely effective at covering your rear or exploiting a main hallway, and as speed and stealth are the name of the game, it is much quicker to have two marines deploying them rather than just one.
• RPGs are wonderfully helpful against mid-late game bosses like Praetorians and Queens. They have a very small but very strong explosion radius, so make sure to hit your target and aim where they’re going to be instead of where they are right now. You don’t need them, but they’re a huge help.
• The sniper, as already stated, is a complete necessity for your Recon classes. It’s the only silent ranged disposal method until the latest late game. It should be researched and equipped ASAP.

4) Detected and Hunting phases need to be kept to a minimum.
• The longer a hunting phase lasts, the higher the aggregation rate grows and you’ll spawn multiple mini-boss encounters. You can end a hunting phase early by welding your marines in a shelter. Do not just wait them out, actively seek to end them if you can.

5) Learn to set a sentry trap.
• Setting a deployable motion tracker with a sentry guarding it can hold the attention of an entire floor while you scamper around in the background for free. Ideal rooms / corridors are long and narrow, with no way to flank the sentry. The noise of gunfire draws humans and aliens alike to this point. Mines to defend the sentry are a big bonus.

6) Don’t be afraid to abuse the save system.
• This game cheats. It will spawn a drone right on top of you multiple times, immediately ruining your stealth and forcing confrontation. You can use shelters and rests as well as elevator rides to trigger an autosave which you can use as a safety net.

7) Keep at it!
• This achievement will be difficult, but those damn bugs can only push you so hard! Keep patient, master each mission, and you’ll come out clean on the other side of the hive!

This is unlockable on NG+ mode which reduces this difficulty to a breeze.

Get resources in the early levels, use these to unlock the high level weapons and unlock upgrades , by mission 4 or 5, you should be fully kitted out and be around lvl 15 with your primary squad of marines.

You still have to play with care and I still spammed the motion tracker near the APC so i managed to stay undetected most of the time but when i got detected my marines rinsed most everything except specials (and even then it just took a special attack or two).

As with the game normally, stress management is key (sergeant's reprimand helps with this) but you can complete levels in one deployment and with 15+ ammo packs.

If you've made it to the end of the game on normal, this should easily be doable on NG+. I finished having lost no marines and on infestation level 3.

this is achievement is currently unattainable, because because every time I defeat the queen on Olduvai, it keeps saying, save failed. A common issue is an insignificant storage space, and will not let me continue the mission. So I redid it again and then it did the exact same problem again right after I opened up the door with the power loader this fucking bullshit dude