Survivalist tip for Aliens:
If you find yourself being fired upon or melee'd by someone from the side or behind whilst performing a TK/SK, or even when you land that killing heavy attack on your opponent, your first thought is to run the hell out of there or try to turn around and attack them. This will just get you killed as there is about a second delay of returning to all four legs and being able to sprint/turn/perform actions again. If you find this happening to you, aim your reticule at a wall whilst still standing on your hind legs and jump. You can jump before the guy you just killed hits the ground. It's faster than running away, makes you far harder to track with a weapon, and you can sprint the second you hit the surface.
When performing an SK or TK, you cannot be stunned until you return to all fours. Chances are that if you have a pred pounding on you from behind and you survive to be able to sprint, the second you do so he's just going to stun you anyway and finish you. While on your hind-legs just as you drop the body, you're free to jump to another surface impervious to stun.
I've lost count the amount of times I've done this, survived only to see the guy that was attacking me, now getting attacked and me being free to jump straight back in there and claim both kills. Works especially well on maps like the Temple, Ruins and Jungle.
This works on the PS3 version. Not sure about others as I've noticed subtle differences like the Pred can be knocked down on PC vers.
Shotgun and Rifle are all you need. Shotgun for short range, rifle for long. As a Marine your melee is the most vital thing to master. Whenever you see an Alien on a wall, or a Pred at focus attacking distance, raise your guard. If they focus attack you it's game-over. If they hesitate it means they're likely going to close the distance for a heavy, as soon as they get close enough, release your guard and whack them one. With the shotgun your reticule will focus straight in on them automatically and there is less than a second's difference before you can pull your double-barrel shot which nets you an instant kill. Also good to note is that the Pulse Rifle has about a half-second delay when using melee. Your light attacks and blocks will be slower. While it is a versatile weapon, being good for short and long-range, it fails when you attempt to melee with it.
Predators are the gods of this game. If you dis-count the Alien's wall-tailing, they are the strongest in melee. They have the highest health, and overpowered weapons to boot. They also cannot be knocked down on the PS3 and 360 (I think 360) other than VERY rarely with the shotgun.
Plasma Caster and Mines are relatively skill-less weapons so I don't really have any tips for them. The disk and the spear, however, take quite a bit of skill to use to their fullest potential. When I play as a Predator I always go for BOTH weapons. And I use them both together. The best thing about the Disk and Spear combo is that you can use them in tandem with your melee. After throwing the disk you can immediately switch to your spear and throw that too WHILST your disk is bouncing around the place. It's important to note that it doesn't work in reverse order, IE you cannot throw the spear and immediately switch to the disk, but that's okay because that's relatively useless. I'm sure many people have noted that the best way to avoid Plasma Casters or Disk throwers when there is no cover, is to actually jump to the left or right. This makes you significantly harder to hit and Plasma shots will often miss you despite their auto-target. When you throw your disk and they jump to avoid, if you can see where they will land, and are very accurate with the spear, you have a stationery target for about a half-second. Spear = Instant kill. After throwing the spear, with your disk bouncing about, most players will be so busy trying to avoid your weapon onslaught that they're wide open for melee. If you're lucky your disk may even ricochet and kill/stun them before you get to them. It's all about keeping your opponent off balance and giving him multiple things to think about and react to.
Never underestimate new players. Lower-ranked players are sometimes MUCH harder to fight than extremely experienced ones. It's the same principle as martial arts really. This is because of unpredictability. Most experienced predators will start with a heavy the second they see you, and will move round to your side. Or they will throw the disk and then focus jump, or whatever us experienced players do. This leads us to get into patterns of dealing with these effective techniques which leaves us vulnerable to the players that will focus jump you and then do 3 light attacks and then a heavy, or a light, heavy, light, light. I've been killed by a lot of new players that got lucky because I expected them to do what I would do if I were in their situation.
Anyway yeah sorry it was so long >.> Hope this helps those of you looking to improve.