Pause the game, hold L + R andenter the following codes:

Restore Health:
Y, Down, A, Up.

Restore Ammo:
A, Down, Y, Up.

Gain 1000 XP:
A, Down, Down, Down.

Level 2 skills:
X, Right, X, Right.

Level 4 skills:
Y, Up, Y, Up.

Level 6 skills:
B, Left, B, Left.

Level 8 skills:
A, A, Down, Down.

The following codes will only work after the game has been completed.

Invincibility:
Y, B, A, X.

Infinite missile weapons:
B, X, A, Y.

Devastating attacks:
B, B, X, X.

All combo upgrades:
Y, X, Y, X.

Small enemies:
Y, Y, A, A.

Slow motion:
Y, X, A, B.

Thanks to Revolution readers Jake, Prince Legolas, Giver of Peace, Nigel Stockdale, Jason, Ant, Matt Springer, Ambien 2000, lotr_xpert and Anonymous!