Meet the following requirements to unlock new Aliens.
Alien - How to unlock
Aerodactyle - Complete Level 2: Mess Hall Mashup.
Big Chill - Complete Level 3: Welcome to Erf.
Bullfrag - Complete Level 2: Mess Hall Mashup.
Cannonbolt - Complete Level 5: Core of the Problem.
Gravattack - Complete Level 4: Underground and Beyond.
Swampfire - Complete Level 1: Stow Away.
XLR8 - Complete Level 4: Underground and Beyond.
