Awesome stuff. I love the art style, and the sound design is pretty good.
The "rainfall" attack is basically impossible to avoid, however. The rest of the attacks are very well communicated, even easy to avoid. But the way the boss is invulnerable most of the time makes it kind of annoying to have to replay. You also can't attack in the air. And you can't start the fight with a gamepad, no button seems to talk to the boss.