When in a fight press L1&R1together then counter attack with R1 (light attack) or L1 (heavy attack).
If you are an alien, get on a wall. If a marine or other enemy is below you press L1 and your tail will kill them.
Easy Way To Kill Quietly
When playing as a predator or alien, sneak up to the back of an enemy and press square. It eliminates them violently, yet silently. Also, if your and alien doing this will some times harvest the weak human fool and grow an alien baby inside of them.