Great gameplay design. It was a bit confusing at first trying to understand what was going on, but I like the idea that you don't want to get your health too high or you also die - that's a new idea I've not encountered before. Also that you can't attack if your balloon sword is fully pumped is neat, but not conveyed to the player very well. It does feel like it's missing sound effects or other visual feedback that you can't attack.
Trying to control the attacks was difficult.
I found it interesting that because you die if your health is too high you have to avoid not just enemies but also the power-ups that normally give you more health, it's an interesting balance.