I really enjoy where the concept and mechanics were taking me, it was like playing rock paper scissors where I was dealt the hand already and just had to make sure I don't land it on the wrong enemy. I think in terms of things to look towards in the future maybe reducing the cooldown would get me more hooked to the game as a whole! I love the idea of every shot being important but each wave still had me going really slow for a shooter. If you want to keep the cooldown to establish a sense of tension maybe make it so that you can fire immediately after you land a bullet, so that there's even more incentive to land each hit.
All in all, great stuff! Just work on the pacing a bit and that should fix quite a few things right up :)