At main menu hold L1 + R1 and press left, up, down, X, right, triangle, and start, then type in the codes:
BACKSTABBER: 1 shot kills.
SNIPER: 1 shot kills in sniper mode.
MIRIYA: Both gunpod and missiles refresh faster.
FLIPSIDE: Flips everything upside down.
SPACEFOLD: Gunpod ammo refresh faster.
SUPERMECH: Invincibility.
MARSBASE: Missiles refresh faster.
CLEAR: Turns off cheat codes.
WEWILLWIN: Unlock all levels.
WHERESMAX: Unlock all models and awards.
MULTIMAYHEM: Unlock all multiplayer levels.
MISSMACROSS: Unlock new paint schemes.