You don't have to hold 'A'. You just have to press 'A' once and then hold 'Z' to shoot. You will exit aim mode if you press 'A' again. I guess I didn't make it clear. I'll make a better tutorial next time.
Even though I already amped up the speed, enemies taking 4 hits to defeat is still too much? I made it only takes 3 hits in the next version, though. That should fix up this issue. You can also take perks to boost your damage as well.