Full health:
Pause, hold all four triggers and press , Down, , Up.

Full ammo:
Pause, hold all four triggers and press , Down, , Up.

Get extra 1000 XP:
Pause, hold all four triggers and press , Down, Down, Down.

Level 2 skills:
Pause, hold all four triggers and press , Right, , Right.

Level 3 skills:
Pause, hold all four triggers and press , Up, , Up.

Level 4 skills:
Pause, hold all four triggers and press , Left, , Left.

Level 5 Skills:
Pause, hold all four triggers and press , , Down, Down.

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

Invincibility:
Pause, hold all four triggers and press , , , .

Infinite missile weapons:
Pause, hold all four triggers and press , , , .

Devastating attacks:
Pause, hold all four triggers and press , , , .

One hit kills:
Pause, hold all four triggers and press , , , .

All combo upgrades:
Pause, hold all four triggers and press , , , .

Small enemies:
Pause, hold all four triggers and press , , , .

Slow-motion:
Pause, hold all four triggers and press , , , .

Thanks to Revolution readers Jonathan1473, bdc, Darling, CaptainUniverse, Kevin Hall, Ed, Paolo Augello, Josh Leonhart, Tupe, and Captain Underpants!