Cheat mode:
Press L + R + X on controller two during game play in single player mode to enable
invincibility and unlimited ammunition. Repeat this code to return to normal.
Alternatively, press L + R + B on controller two during game play.

Toggle enemy AI:
Press L + R + A on controller two during game play in single player mode.

Toggle enemy AI and unlimited ammunition: 
Hold L + R + A + B on controller two when the game is loading or starting. 

Glitch: Invincibility:
Intentionally die just before entering the next level or section of the current level. The
best place to do this is when you come across the third power plant on the first level,
where destroying it will takes you to the next level. Find where the nearest street to the
third power plant is located. There should be a pit here. Turn your back to the pit and
destroy the power plant. After it explodes wait for approximately two seconds then
jump into the pit. If done correctly, the mission completed mission will appear followed
by a message stating that you died. When the next mission starts, you will have no life,
but will be invincible (except for falls). Note: Although the game will continue normally,
you will not be able to save since you are considered dead.

Play the game in the first half of level 1. After destroying the first generator, keep going
until reaching the door to second half. Go until you cannot move further, but press X +
Y + A before loading sign appears. The dead sign will appear. Do not fire. The game
load after that with 0 on energy. You now can fire, but will be invincible except for falls.
However, you cannot save the game.