Cheat Mode (Required for other cheats)
At the Password screen enter Left, Up, X, Square, Down, Triangle, Square, Down. You can now enable infinite everything (lives, weapons, etc.), play as a monster, and choose your starting level.
Baby Monsters
Enter the CHEAT MODE code then, while still on the title screen, press L2, L2, R1, R1, L1, R2, Select. You will now see a new option titled 'Reduce' when you start the game.
Blurry Screen
Enter the CHEAT MODE code, then press L1, L2, L2, L1, L2, L2, L1, Select.
Change the Music
Enter the CHEAT MODE code, then press L1, L1, L1, L1, R1, R1, R1, Select. While playing, pause the game, go to the OPTIONS screen and you'll see a new option called Current Track. Highlight it and press LEFT or RIGHT to change the music.
Debug Mode
Enter the CHEAT MODE code, then press L1, L1, L1, R1, R1, R1, L2, L2, L2, R2, R2, R2, Select.
Disable Combos
Enter the CHEAT MODE code, then press L1, R1, L2, L1, R2, Select.
One-Hit Kills
Enter the CHEAT MODE code, then press L1, R1, L2, R2, Select.