Very cool game idea. I like the style a lot. The stages are well made and intuitive. It was directly obvious that a transparent boss won't take any damage (but I still tried).
I also like to give some constructive feedback:
On a german keyboard it is the z key is at the place of the y key. An alternative would've been nice.
May you also think of having checkpoints after each boss-stage. So that one doesn't have to start from first stage again.
Very nice game :-)