To achieve the Bronze Cheats, you must end the game in the easy difficulty This will open two options for multiplayer mode: Weapons Stay and One hit kill
NOTE: Remember to save the cheats on your memory card
To get gold you must beat the game on hard. Winning gold will give you infinte ammo, all weapons and all the rest of the multiplayer codes you recieve when you win silver and bronze.
To enable Silver Cheat, finish the game in MEDIUM difficulty. This will open 2 more bars in multiplayer mode : Game Speed and Blast Force. Plus it will unlock the multiplayer cheats you get for winning bronze.