Enter the following codes during gameplay.

AA: Up, Y, Right, Y, Down, Y, Left, Y, Up.
Enemies don't target you: Down(3), LB, Down(3), RB, Down(2).
Invincibility: LB, Up, RB, Up, LB(2), Up, RB(2), Up.
Kill: LB, Down, RB, Down, LB(2), Down, RB(2), Down.
No clipping mode: Up(3), LB, Up(3), RB, Up(2).
Refill ammo: Y, B, A, X, RB, Y, X, A, B, RB (an ammo cartridge will spawn).
Restore health: Up(2), Down(2), Left, Right, Left, Right, B, A (a medpack will spawn).
Unlock all levels: Left(4), LB, Right(4), RB.
Wireframe cycle: LB(2), RB(2), LB, RB, LB, RB, Left, Right.