On most revisions of Galaga, there's a trick that stops enemies from firing at your ship. In Stage 1, kill all aliens except the two bees in the bottom-left corner. Allow the bees to dive down and shoot at you, while dodging their shots. After around 15 minutes or so, the bees will continue to drop down, but they won't fire. Let them drop down another 5 or 6 times, then kill them. After all this, enemies will not shoot at you for the rest of the game.