Revival Glitch "Zero HP"
This will work with any character and beneficial for OFFLINE play. Just before your character die (indicating with a moan), stand closely near the doorway (or stairway) and wait til your HP reaches zero. You should be able to move in and out of the room as if you're alive with HP (health bar remains at zero). Once you enter back into the room or area all the enemies in the particular area respawn back and won't move nor attack you no matter what you do. Can be use on target(s) in close quarter area, with multiple enemies, and pesky enemies that revive quickly.
Unlock Bonuses
Extra stages (10 stages 2 per area) - Finish all 30 stages.
No character restriction - Finish all single player Missions.
Use any weapon for any character - Finish all single player Missions.