Invincibility:
Press Square, Circle, X, Square, Circle, Square, X, Circle, X, Square, Circle, X at the Title screen. Enter the Cheats section at the Sage, and the last card will be Invincibility. Activate it with X.

Health pick-ups give 100% health:
Press X, X, X, X, Square, Square, Square, Square, Circle, Circle, Circle, Circle at the Title screen.

Infinite arrows:
Press X, Circle, Square, Square,, X, Square, Circle, Circle, X, Square, Square X at the Title screen.

Stronger opponents:
Press X, Circle, Square, Square, X, Square, Circle, Circle, X, Circle, Circle, X at the Title screen.

Weaker opponents:
Press X, Circle, Circle, Square, X, Square, Square, Circle at the Title screen.

Disable arena AI:
Press X, Circle, Circle, Circle, X, Square, Square, Square, X, Circle, Square, X at the Title screen.

Weapon select:
Find each Tuku on all levels to unlock an option at the Sage that allows you to use any weapon on any level.

More costumes and art:
Beat the arenas by completing all challenges for both time attack and body count to unlock extra costumes and art.

Unlock arenas:
Beat all of the challenges in a level to unlock that level's arena.

Thanks to Revolution readers derrick hendricks, Daniel Hoffman, Slicer, Lewis, and Marc!