When you reach level 3, fight your way through the stage until you get to the mini boss after you climb the building, "The Flying Squito Nest". 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 possible and have him facing the right. Then, tape down the Y button and turn on your turbo mode. Set your controller down and come back in a few minutes to get a lot of lives (your lives max out when they get to 29). If you don't have a turbo controller then you must press Y repeatedly.
There are two places on Level 1 where your character can remain motionless to collect extra points and lives. After starting level one, your character will find a futuristic car. Instead of destroying it, jump over it and use it as a shield against the attacks that originate from the left side of the screen. Keep firing towards the right side of the screen, instantly killing all opponents that appear from that direction to collect easy points and the occasional extra life. Keep going on Level 1 until your character starts to collect homing missiles. Take the missiles to the next higher platform, and out of the way of the ground activity. Use the missiles to destroy all enemies that come into range to earn more points and lives. Note: When playing under the highest difficulty setting, a maximum of twenty-nine lives may be accumulated.
Play the Normal or Hard difficulty setting and reach level 6 and the final Boss. Notice the different shapes moving around it. Shoot the one that resembles a spike and various guns and bombs will drop. Get under the Boss and wait until the gun marked with an "M" appears, then collect it to get the mini gun.