It took me a while to understand how the firing works. The mechanics is similar to galaca; you can have only two bullets on the screen at a time. Of course, the PterPod (green one), PterBoom (purple one), and FireBall(red(Orange?) one) shoots multiple shots at a time, excluding the pod, which shoots a laser at max level. To shoot faster, you would want your bullets to disappear as fast as possible (either by leaving the screen or hitting an enemy) until one or none remains. To maximize this, try to stay in the middle of the screen so you have enough time to react to enemies but gives your shot enough time for your missed shots to leave the screen. When you progress later in the game and too much enemies are heading your way, you can start to move lower, just be careful for the grarrl that could from the bottom.