On the first level, wait until the enemy ships are in their proper formation. Shoot every last one except the one in the bottom-left corner. Now move your ship to the right corner. Let your enemy dive at you and go around until he doesn't fire during his pass. During the pass after not firing, shoot him down if he still isn't shooting in the next pass. If you do this correctly, the enemies in the rest of the levels will not fire at you for the rest of the game.