Cheat Codes   Imput each of these codes on the pause screen while playing to enable their effects. You must also hold L and R throughout the entire code for all of them. Directions are always done on the D-Pad as well

Restore health : Y,down,A,up
Restore ammunition: A,down,Y,Up
have all level 2 upgrades: B,Right,B,Right
have all level 4 upgrades: Y,up,Y,up
have all level 6 upgrades: X,left,X,left
have all level 8 upgrades: A,A,down,down

For the following codes to work you must have beaten the game at least once.
Invulnerable(invincible): Y,X,A,B
Infinite ammo: X,B,A,Y
Always devastating: X,X,B,B
All combo upgrades: Y,B,Y,B
Small enemies: Y,Y,A,A
Slow mo: Y,B,A,X
(Thanks to Glmblue@cox.net )