Hold Up + L + R, and press Start at the Konami screen.
Walk through the first mission without shooting or hitting any targets. Your score should be 0 points. Defeat the Boss without shooting its side gun turrets. Your score should be 2800 points. When the asteroid level begins, press X to change weapons until the unlimited napalm gun is selected. Survive this mission to keep the unlimited napalm for the remainder of the game. Note: It cannot be powered up.
There is a new, hidden weapon in this game called Napalm that will allow you to take out enemies in no time. First, you must do a walk-through of the first level. DO NOT shoot anything until you get to the first boss.
Destroy the first boss, but don't shoot either of the side guns. You must come out with 2800 points when your mission is complete. At the beginning of the second mission, you will be flying through a field of asteroids and enemies. If you get destroyed here, you will lose the Napalm weapon and will have to do the trick all over again. You will see that it is very easy to destroy the second boss using this new weapon. From here, you will be able to continue, and keep this weapon.
With this generous trick, you will be able to have an increase in credits up to 6 instead of the normal 3! First, wait until the Konami logo has passed, and then at the demo before the title screen appears, press and hold these buttons: L button, R button, and UP on the pad. Then, press START. Keep holding these buttons when the title screen appears and press START again, and do this once more when the Game Start option is highlighted. Let go of all buttons and play past the first level to see your credit gain!