I like the main mechanic and the graphical style is quite nice. Its a prototype so its clear that there are aspects that haven't been added yet (like the audio and a proper menu). But overall I think the concept is pretty solid. It would be nice if there were save points so that I could keep playing and seeing further levels, but I understand that your probably intending that as being part of the challenge. Maybe you could group a set of dungeons together in sections that can act as save points?
I like that you can block attacks with your attack and vice versa. The computer AI is also interesting, them being able to bank shots its pretty cool.
Overall its a cool concept. I'd like to see more of a variety of creatures and shot types (multiple shots, curving shots etc).