Very cool concept, deserves to be expanded beyond just one boss. I love this blend of a space shooter/bullet hell like game that uses moon physics rather than giving you a full range of motion, it made me feel that much more skilled when I managed to take down enemies.
In truth I don't know how much the numbered countdown is really needed, just having to kill the boss before I splatter against the ground fits the theme enough in my opinion!
It'd be awesome to introduce more movement types, like grappling off falling obstacles, or even deflecting attacks back at the enemy. It just seems like there is so much potential to add things like combos and flair.