The game mechanic is quite unique, but hard to get at first. I died several times to realize I need to let one enemy alive, then I understand the title lol. Maybe rephrase the tutorial a little?
I noticed a bug in the tutorial level that you can shoot to the lower door and the bullet will come from the above one. I thought it was a feature at first and I think that would be fun in a way too if we have that for left and right.
Another thing I haven't figured out is what is the bonus... I assume it should make my bullets stronger but I didn't notice that. maybe a clear explanation of what ability you get would be helpful.
But all in all an interesting game, the control is very smooth, I'd like it if there are more sfx but consider the time limit it is good enough!