EDIT:
Aliens=
Attacking:
S1.
Dretch= Use headbites constantly. Always keep moving, use wall walk, don't jump. As pointed out by Vai, hit them in the leg first, as that will only bring them down a few hp, enough so that a headbite will kill them, but not enough to make them use their medkit.
Basilisk=The best strategy is to sneak up from behind and stick them. (Note, you can jumps on their head, and look at their head and they get stuck) After sticking them from behind, jump constantly and swipe at their head. Use head swipes. Use wall walk to get behind the human(s). Another trick for basilisks is to use "head jumps" what this is, is when you get in front of them, jump over their head, and swipe at their head. Than do the same from behind. Repeat as many times as necessary.
Marauder= Use jumps, to jump of walls and avoid getting hit. Jump on and off of their head while chomping it.
Dragoon= Use pounces!!! The deathtrap for a dragoon is running at a human who is shooting it and dodging. So you must pounce almost all of the time to avoid fire, attack the enemies form longer range, and get setup to attack. Pouncing is the way to win with dragoons. (Note as the biggest enemy, humans will most likely concentrate their fire on you, so be careful to monitor your health wisely, or else you will die.)
Basilisk and Dragoon= If there are a few humans, the basilisk just runs around behind them and touches them from behind (so they can move) while the goon walks up and chomps them in the face. Do this for all the humans. Note the dragoon distract/kills humans so they don't notice basilisk until it's to late.
Marauder and Dragoon= Works best in corridors with tall ceilings. Humans moving in packs (it works fine on 1 human also) than before they round the corner, marauder jumps out and jumps as high as possible, than bouncing of walls and taunting (f) to draw humans attention. Dragoon than attack only in pounces, than retreating. Pounce-retreat,pounce retreat. Meanwhile the marauder is jumping around like crazy getting closer and closer to them. (Note it is ok, for the marauder to take a rip at the humans' head.) Affective against large groups of humans with skilled Dragoon.
Dretch and Marauder/Dragoon= This one isn't to much of a strategy, just noting how annoying it is for humans, when they are chasing after a dragoon to also have to deal with a couple dretches bouncing at their heels. This is much, much more effective if the dretches know how to use ground head bites.
Multiple Dretches= This is a good strategy to use right at the beginning of the game, on a smaller map. Especially helpful if humans rush also, farther they have to run the more chance you will have to kill them. Works best with at least 4 dretches, but can be done with even 2. I'll explain how to do it with 4, and you can mess around with it for 2 or 3. So, for 4 dretches: The first one runs on the ceiling, than one on each wall, and lastly one on the ground. The one on the ceiling runs past the human(s) and than drops onto the ground to attempt to hit them from behind. The two dretches on the wall than drop off next to the humans, and attempt to headbite them, while moving in circles at their feet, so they are hard to hit. Lastly the ground dretch just runs right at the humans, drawing their fire, and dodging it, if this dretch manages to get to the human(s) than they to can attack.
S2
Pretty much the same as S1 due to the aliens not getting any true new classes. (upgrades but no original ones.) Some notes:
*For Marauder and Dragoon, an advance marauder is much more effective at this for two reasons. 1 I believe that advance marauder are able to jump higher/farther. 2 The advance marauder zap works quite well in this situation, (marauder and dragoon.) because it enables you to be able to hit them from farther away, and you can also hit multiple targets at once.
*For Basilisk and Dragoon, Have the advance basilisk running around and gassing the humans also.
S3
Advance Dragoon= same as dragoon, but you can be a little braver. Use barbs to take out key buildings in sudden death like the armory.
Tyrant= Chomp everything.
Advance Dragoon and Tyrant. This is the ultimate base killing duo, The tyrant charges in, while the advance Dragoon snipes from a distance. These guys are so tough there is not much need for any real strategies. But there are a few.
In Sudden Death, the advance dragoon should go for items like the armory and defense computer, and even telenodes if you are a good enough shot.
Tyrants should go after the turrets.
Humans
The overall strategy for humans is= Work in groups! You are so much better and more effective if there are multiple teammates there with you.
S1
Rifle= A very good weapon, and you get it for free. Best used in groups, 5 shots will kill a dretch, as each pellet deals 5 damage. Fire in short bursts. Holds 6 clips of 30 each.
Pain saw= Although skilled players can use it to kill dretches and dragoons, this is a base attacking weapon. Works well if teammates cover you from enemies, so that you can get to the base at full health. Has unlimited ammunition. With a painsaw, run past acid tubes, and go first for the eggs, and than the OM. Use medkit when needed.
Shotgun= Close combat weapon. At point blank range it deals around 56 damage. It is semi-automatic and has 3 clips of 8 each, so don't waste your ammo at long range. One good strategy against dretches and basilisks, is to jump over them, and shoot them while you are on top of them. (Note aliens have no weakpoints.)
Las Gun= Deals 9 damage per shot, holds 200 "cells" shots, and no clips, or 300 with battery pack. Has almost perfect aim, even at long range. (Note this weapon is inst hit, that is the split second you pull the trigger, it hits where you are aiming, same with shotgun and rifle.) 3 shots will kill a dretch, 45 will kill a tyrant.
Mass Driver= The sniper rifle, hold z to enable zoom, deals 52 damage. Good if you have a teammate with an automatic/close range weapon (excluding painsaw) near you. To hit dretches, aim a little in front of them, than fire, and you can kill them, when they are dodging, see if you notice any pattern in their approach. Also note that the dretch hitbox is slightly larger than the model. What this means, is that the area where you can deal damage to it is a slightly greater area than the model (the picture of the dretch) If you shoot right above them you should hit.
Chaingun= pretty useless until s3 with battlesuits. Crouching lowers the spin. This is a good support weapon, for bleeding tyrants, or weakening dretches, and the like, but should not be used alone unless you have a battle suit.
S2
Pulse Rifle= Holds 4 clips of 50, 75 with the battery pack. Against dretches fire somewhat randomly at whatever surface they are on. 3 shots should kill them. Another good support weapon, although this can be used on its own. (Note that the slow traveling speed make it hard to hit marauders and chase down fleeing enemies.)
Flame Thrower= Spits 150 seconds of flame, very effective against dretches, good in groups with other weapons. Two experienced flamers together should have no trouble taking down a dragoon in small areas. Do NOT use in vents, do not spin around while firing.
S3
Lucifer Cannon= The deadliest weapon when used right. Against dretches, do not fire rapid small bursts, instead power up a large shot, and shoot at them on the ground, the splash damage should kill them if done right. A good combination with battlesuit and chainguner to bring down tyrants.
Overall strategies for humans
*The rolling v= human assemble into a "v" in that shape, mass driver goes on point, (in the rear, this is and inverted goose flying thing.) than Pulse rifle, than Chaingun, than flamer or shotgun. When A larger alien attacks one of the guys on the side, in the front, the guys just peels away, and moves to the back. When alien goes for the middle they , just keep backing up until you break and rejoin formation on the other side of the alien. Keep moving so they can't hit you. For dretches against rifles, form into this position, and all fire in your straight line (eg. right in front of you.) This should create a virtual screen of bullets that should have the smaller aliens dead in no time.
*The Moving screen= this seemingly random attack should take care of most enemies. Form up against the walls, men on both sides. When an alien attacks, keep shooting until he is very close, than when you can see who the alien has decided to attack, that person runs toward the rest of the people, while everyone else shoots the alien. Repeat.
That's all I'm going to put down now, I won't go into complete detail or anything. I hope this helps.
Poll