I got it too shoot, but it won't consistently work. Also it takes a lot of hit to kill one guy and moving while holding a and z is pretty difficult. I'm going to chalk this up to I'm not very good at your game.
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.