Get a gold medal rank on the indicated level to unlock the corresponding cheat in an apartment sub-menu. You cannot progress in the game if any cheats are enabled.

Crazy Deaths: Lucky's Bar level
Gun Splitters: Crack House
One Shot Kills: Ryker's Island
Unlimited Ammo: Pier 74 Revisited
Invincibility: Every level
No Reloads: Stark Towers
Unlimited Slaughter: Takagi Building Level