In Game Cheats

These cheats are to be entered during the game.  And you can still complete the mission successfully!
 

Invincibility (enter during the game) L+D down R+C right R+C up L+D right L+C down R+C up L+D right R+D down L+D left L+R+C right All guns L+R+D down L+C left L+C right L+R+C left L+D down L+C down R+C left L+R+C right R+D up L+C left Max ammo (not infinite) L+R+C right R+D up R+D down R+D down L+R+C right L+R+D left R+D down R+D up L+R+C right R+D left Invisibility (single player) R+C left L+R+C up L+R+D left L+R+D up R+D up L+C left R+C up L+C down L+R+D left R+D right Make certain player have ghost mode until they die (multiplayer) L+C up L+R+C left R+D up L+C right R+C left L+D right L+R+C left L+C right L+D up L+R+C down