Rebel Kronn warrior:
Shoot the first three S.T.A.A.R. team members and nothing else at the beginning of the game. After the first human is shot, you will not be harmed by any gun fire. After the third human is shot, you will transform into an alien in the main hallway of the first level. You will have infravision when playing as an alien, which allows targets to be more visible.

Hint: Baby Aliens:
When you enter the building from the helicopter, shoot the exit sign when you see a purple alien, then shoot the first three lights. This gives you the opportunity to shoot baby aliens before they form.