What is the best strategy for being an alien?

Here's a basic outline of Xeno tactics you should learn:

Learn the melee combat system and then own everyone.

Learn the basic counter, block then light attack..

Learn how to block Predator light attacks, by understanding the differences in attack time between light and heavy attacks.

Learn how to perry a Predator heavy attack, again by learning the differences in attack time. simply light attack to disrupt a heavy.

This one is hard: Learn how to pump fake a Heavy attack, to confuse a skilled Predator. start heavy attack, if he's decent he'll try to light attack and perry your heavy, when you see him do that, block instead and and counter him and then heavy while he's stunned.

dead meat.

For Marines, basically never stay on the ground. always bug walls and ceilings.

In tight corridors stay on ceiling and walls and do consecutive rapid heavy attacks, kills both Pred and Marine in two hits.

Attack from above. Pounce a marine and then slap him with a single light attack and he's dead.

learn the dynamics of a basic combo that kills a Marine in two hits. run up to Marine, light attack then back up and run to his side while charging heavy attack, he won't be able to perry heavy attack because he is stunned temporarily by light attack. dead meat.

or you can run up to him, from a direction he's isn't looking at and heavy attack first and then slap him with light while he's stunned.

practice these strategies for a couple of hours and you'll be at the top in kills every single time.