This guide was written specifically to help people (and myself) understand the AI's effectiveness and bugs in Aliens: Colonial Marines.

First off, let's discuss difficulty scaling. The AI in Colonial Marines is a scaling system. The gist is, the more you play, the harder the game gets. Be aware, some enemies that spawn in the campaign are purposefully weaker than usual, and so are not affected by the scaling completely.

As for what the actual determining factor is? From my tests, it's simply based on kills against AI enemies. These kills seem to be somewhat persistent, because you can die and they'll still count across all gamemodes. But it also seems to be temporary. Something resets this count to zero regularly, maybe per day. Editing cfg files can also reset this.

The effects of the scaling are most visible in coop, enemies will have more health, react faster, be more aggressive, etc. Whereas in singleplayer, even on ultimate difficulty the scaling cap is so low that it's still not as hard as I'd like it to be. It seems to be based on number of players as well as number of kills, so the more players, the more multipliers involved. One player, if they've been playing singeplayer for awhile beforehand, can majorly increase the overall difficulty of a coop game. This has been tested several times by myself, and best I can tell may also affect bughunt.

You can "farm" kills to test this scaling. Just play a hard part with a lot of enemies, kill a bunch, die, kill a bunch, die, and so on. The effects wont apply until you die and restart the checkpoint, but each kill you get before then is relevant. I've done this a lot on Distress to test the effects, and it does indeed work.

As I said before, there is a cap to this amount, so eventually it'll stay at the cap and not go any further. But note, this cap has a much more vibrant effect in coop, so be careful. Your actions can make a coop game extremely difficult just from doing this in singleplayer beforehand. While this might sound like a good thing, bullet spongey enemies that down you instantly isn't for everyone. Especially if they use the TemplarGFX mod, which gets a little stupid with too much scaling.

It's somewhat commonly known by now that there's a typo in the PecanEngine file located here:

Documents\My Games\Aliens Colonial Marines\PecanGame\Config

This line specifically:

ClassRemapping=PecanGame.PecanSeqAct_AttachMarineToTether -> PecanGame.PecanSeqAct_AttachPawnToTether
ClassRemapping=PecanGAme.PecanSeqAct_AttachXenoToTether -> PecanGame.PecanSeqAct_AttachPawnToTeather

It's believed this affects AI pathing, because there are tethers that keep AI aware of their area. So in theory, fixing this typo improves tether pathing. Some claim it helps with xenos when wall-climbing.

It's not really a huge game-changing difference as far as I can tell, but most people prefer to fix this typo in the xeno line, along with the capital A in PecanGAme.

ClassRemapping=PecanGame.PecanSeqAct_AttachMarineToTether -> PecanGame.PecanSeqAct_AttachPawnToTether
ClassRemapping=PecanGame.PecanSeqAct_AttachXenoToTether -> PecanGame.PecanSeqAct_AttachPawnToTether

If you also change this typo in your install folder's DefaultEngine file under PecanGame, it makes the fixed version the new default. Be aware though, editing this file tends to make all video tweaks revert to default unless the PecanEngine file in My Documents is read-only.

A lot of people talk about tweaking the values of the memory budget in the PecanEngine file, and how it may have an effect on the behavior of the AI. Here is that section fully default.

[MemoryBudgets]
LocationMB=1
ArchetypeMB=1
ScriptMB=1
SkelMeshMB=2
StaticMeshMB=10
TextureMB=120
AnimationMB=15
AnimationSeqMB=1
AudioMB=10
ParticleMB=5
ParticleTemplateMB=1
LightingMB=1
AmbientLightingMB=1
LevelObjectsMB=4
BSPMB=1
KismetMB=1
TerrainMB=4
DecalMB=1
NavigationMB=1
PhysicsMB=10
ScaleformMB=8
EpicUIMB=1
MaterialMB=1
GameDataMB=3
ShadersMB=10
FragmentationMB=1
EngineCoreMB=1
SceneRenderingMB=1
GPUMemoryMB=1
FoliageMB=1
BinkMB=1
StackMB=1
CodeMB=30
BehaviorsMB=1
OtherObjectsMB=1

From my testing, even if these tweaks do have an effect, it's so negligible that you might as well not bother.

There seems to be a pretty common bug in certain areas where a xeno will get stuck completely, sometimes fully unresponsive to damage. I've tried to fix this bug with various experiments, including using mods created by other people, as it can sometimes halt enemy spawning or the mission until this alien is killed. Unfortunately, I don't think anybody has had any success.

If you run into this bug, you might have to restart the checkpoint in extreme situations, but if the alien is killable, killing it should fix things. This is a totally "luck of the draw" situation, you can't predict it happening- sometimes it does, sometimes it doesn't. It happens a lot during the major battle of Distress in the hangar, where you have to protect the barricade. An alien will get stuck on the right wall of the hangar, up near the vent where they spawn. It'll sit there forever, but you can kill it, and the spawning starts naturally again. Other times an alien will get stuck in the ceiling near the dropship, you can hear it but you can't kill it. This is the worst case scenario. A lurker at the end of No Hope at Hadley's can occasionally get stuck as well. It makes typical lurker sounds, but it's stuck unresponsive on the ceiling.

It happens, often fatally for the game, in bughunt mode. Sometimes the alien breaks out of it's funk with time or by reacting to a player, sometimes it's sitting in a place idle and can be killed (listen for the typical xeno sounds), but other times it might be outside of the level limits idle in a place that it can't be hit. In that case, you've got quite a situation. This might require restarting the whole game, which is totally awesome for everyone involved. Don't be too hasty though, wait to see if you can find the alien first or break it out of the idle bug... a lot of times you can save the day by killing this problematic alien. If that fails, try to activate a threat supressor, which will kill all aliens currently on the map when it completes.

The AI in Colonial Marines might not be the most outstanding enemy AI in gaming history, but it's definitely got some mysteries to it. Mysteries that I wish I understood more, because nobody in existence (at least on the internet) has commented on them. So, stay frosty my friends.