When you get to level 3, fight your way through the stage until you get to the mini boss "The Flying Squito Nest" (the mini boss after you climb the building). Make sure you have the machine gun as your weapon or this won’t work. Position your character as far to the left of the screen as you can and have him facing the right. Fire down and after a few minutes you will get a lot of lives (your lives max out when they get to 29).

Beat the Game on the Hard Difficulty Setting to view a special ending not seen after beating the other two difficulties.