All of the following cheats will only work if you have beaten the game (you can try before-hand, but we don't recommend it).  Once on a level, pause the game and hold down all the shoulder buttons, then imput the following:

Always Devastating: Press Square, Square, Circle, Circle.

Small Enemies: Press Triangle, Triangle, X, X.

All Upgrades: Press Triangle, Circle, Triangle, Circle.

Invulnerability: Press Triangle, Square, X, Square.

Slow Motion: Press Triangle, Circle, X, Square.

Infinite Missles: Press Square, Circle, X, Triangle.