I wish I had these Rainbow Six Extraction tips when I started playing it. Extraction is a game with rules that don't apply to many other shooters, so it was very easy for me to trip up early trying to play it like Left 4 Dead. Instead, you have to pay Extraction like a light stealth game where patience is rewarded and headshots are basically mandatory.
One of the first instincts I had to unlearn was to kill every enemy I see. Most Archaeans will die to just a few shots, but open unnecessary combat usually isn't worth the risk of alerting the stronger enemies nearby. Subtlety is key, which is a nice change of pace from the other big co-op shooters around.
Here are my best tips for surviving runs (both solo and with a team) in Rainbow Six Extraction.
Crouch almost everywhere
It can be a little dull to sneak your way down every hallway, but since you want to minimize the chance of kickstarting a horde with a spooked Archaean, staying low is the way to go. From a crouched position, enemies will stare at you for a long time before actually "seeing" you, during which time you can score an easy headshot or simply move on to another room.
Crouching is also best when you're trying to sneak up for a melee takedown against the more skittish enemies like the Smasher and Tormenter.
Destroy nests before completing objectives
It is almost always a good idea to scour a new map and destroy every nest you can find. Not only does this clear a lot of Sprawl from the map, but it will also keep more Archaens from spawning if you happen to trigger a big fight. This isn't just a convenience, either—you will alert an enemy at some point and letting alerted nests go unchecked is the fastest way to get killed.
Prioritize healing over gadgets
Listen, Sledge's hammer is cool and Pulse's scanner is handy for finding nests, but you should really prioritize healing gadgets when going into a higher difficulty mission. Doc and Rook are great picks for instant survivability. Finka can also restore some health over time once upgraded, but otherwise her health boosts only last a few seconds.
Depending on which of these health-oriented operators you don't take, you can replicate their effects with secondary gadget slots. If there's no Rook, it's probably a good idea to wear body armor. If there's no Finka, it might be smart to have a self-revive kit.
Break line-of-sight with enemies for a do-over
If you've alerted every enemy around and you're getting blasted from every angle, there still might be a chance to break away back into stealth. Never hesitate to sprint away from a messy fight. Once you've put some distance between yourself and the horde, hunker down in a room and take stock of what's still chasing you. If it's just a few Grunts or Spikers, you can probably take them out with a few headshots and go back to relative stealth. The catch is re-entering stealth doesn't deactivate aggravated nests, so enemies will continue to spawn.
If too many enemies spawn, just leave
There will probably be several times in your Extraction career that the nest situation has gotten out of hand. Aggravated nests will continually spread Sprawl and spawn new enemies with virtually no limit (at least not one I've been able to notice). This can be really bad when a bundle of nests have gone unnoticed for a few minutes and produced way more enemies than a few operators cant handle at once.
In my experience, this can happen very fast during Aberrant Nests missions (the one where you have to destroy green nests). Green nests are often clustered very close together in a group of nine or ten, and just a single Archaean scream can trigger them all. After wiping my squad several times, I realized it's better to just leave in these scenarios, either through the airlock or extracting from the mission altogether. You're unlikely to destroy every nest and hold F on the epicenter without getting ripped to shreds by 30 aliens, so you might as well take the XP hit and not lose the operator.
Impact grenades are the best grenades
Many of Extraction's grenades are basically different flavors of stun grenades. You can slow Archeans down with Glue, daze them in place with a flash, or blow them all the way up with a frag, but I've yet to use a grenade as useful as the Impact. Not only can it create an instant hole in a wall handy for quick escapes, it can clear massive areas of Sprawl goo, destroy nests, and stun enemies long enough to do a melee takedown from behind.
Impacts are the perfect companion to almost any situation in Extraction. At least one person should have them on your squad, and that person should have first dibs on REACT tech refills when found.
Suppressors are (almost) always worth it
There's a reason every gun that can use a suppressor in Extraction has one by default. On 99% of runs, you'll be glad you had a silenced gun to covertly shoot nests and unaware enemies without alerting the whole complex. Sure, detaching the suppressor comes with a damage and recoil boost, but since most enemies can also be killed with a single shot to a weak point, you're not missing out on much.
That is, except, when it comes to heavily-armored enemies like the Apex and Smasher. There's no way to one-shot-kill these jerks in a straight up fight (Smashers can be stunned and stabbed in the back), so killing them faster when an unsuppressed gun is worth making more noise. You'll only encounter these harder enemies in significant numbers on Critical or Severe difficulty. Consider bringing one or two loud guns between your squad and keeping a silenced backup for stealth shots.
Don't stay on the lower difficulties for too long
Once you get the hang of sneaking around and headshotting grunts in Extraction, the lower two difficulties can get pretty darn easy. It can seem daunting to jump up to Severe or Critical, but you can survive anything with good teammates and a bit of coordination. The game is clearly meant to be played at these higher levels—you won't even see every enemy type that exists in the game until you play on Severe. It's also worth it for the extra XP. Once you get to Severe, you'll easily level up an operator several times after one successful run.
Be aware of how missions change in Solo
Extraction scales in some pretty important ways when you play it solo. Not only do solo players get a free 50 health boost at the start of a run, but the objectives themselves are modified to be more doable by a single person. For example, a solo operator only has to defend one planted bomb, or plant trackers in three nests instead of five, or stealth takedown a single Archaean instead of three.
The strangest mission tweak by far is with MIA operator rescues. When played solo, there are fewer tentacles to fend off and they tend to spawn very close to the captured operator. This makes it super easy to pull the MIA operator for a second, step off, and shoot every exposed tentacle node.