Start a mission as normal. When you transport in, pause the game to bring up the sub-menu. Highlight Review Mission, then, hold T, press C,S,S,C, release T, then hold X, press T,T. The word Cheater should appear at the top to confirm the trick worked.
Invincibility
Start a mission as normal. When you transport in, pause the game to bring up the sub-menu. Highlight Review Mission, hold Right, press C,S,T, release Right. Then hold Left, press S,C,X, release left. The word Cheater should appear to confirm the trick.
Maximum Health
Highlight Review mission, hold square, and press circle, x, triangle. Release square, hold circle, and press square, x, triangle. The word Cheater should appear at the top of your screen.
Refill Ammo
Start a mission as normal. When you transport in, pause the game to bring up the sub-menu. Highlight Review Mission, hold C, press S, release; hold T, press X, release; hold C, press X, release; hold X, press S, release. The word Cheater should appear to confirm.
Secret Level
In the Throg Breeding Facility you'll find a secret entrance to the "Lost City". Look for it in the lower right-hand corner of the map. You'll need to break through three or four secret doors to find this exit, but it's well worth it.
Cloak...