I really liked this game. It was fun and using hp to shoot was fun and interesting.
However, I'm not exactly seeing where the "less is more" is in this game. You don't gain anything from having less health or less of anything. Additionally, charging up the attack didn't seem very worthwhile. It was mostly just a slower way of attack. It costed the same amount of health and took a similar amount of time to charge it. If it were noticeably more damage efficient, health efficient, and/or time efficient, I would make more use of it.
Otherwise, the gameplay is really solid and the game is well polished. The visuals were pretty bare bones but the game was very clear and communicated itself really well.
The boss fight was pretty cool. You're able to progress it as quickly as you want.
I really enjoyed this. Great job!