Invincibility:
While in the game:
Hold the Trigger (L) and press (X) (Y) (B) (A)

Note: After each new checkpoint you reach make sure you re-enter the code, it deactivates after each check point. Can not use until you first pick up the pistol/armor from the cabinet the AI Soldier unlocks.

Flashlight & Couch Tips:
The flashlight can be a lifesaver. As the harder the level is the more ammunition you will need, so use the Flash Light 'slap' as much as possible, use it on all the zombies (without guns) While holding the dash button (Left Trigger), run into them and hit them once (right trigger) and quickly back away, then repeat, this will kill the regular zombies without getting hit (be sure to back away quickly though or you will be hit). Also use the Crotch to avoid Enemy Plasma balls/rockets/hits, it often will cause the AI to miss unless they are using the machine gun/Chain gun.

God mode:
After about the third mission you can use the god mode. Just open up your pad and hold the left trigger and press X,Y,B, and then A. Then your pda will close you might have to pick up a health pack to get it to start working.