Very fun game and upgrade tree. The boss fight was a little frustrating, but once you get all upgrades, it's just a matter of beating the game. I'd love to see multiple 'worlds' with different enemy types and bosses for each. This game is certainly worth the playthrough.
So sometimes it would seem like the boss would hit a wall in a way I wouldn't expect (because of it's large hitbox). When it would do so, its forward line would complete instantly and kill me. It would mostly happen when we were both near an edge and it would charge at me nearly parallel to the edge, but slanted just enough to hit the wall and cause the said effect. It didn't seem like it was an intentional mechanic and I compensated by only fighting well within the borders. Perhaps it was a skill issue on my part, but that's just my perspective. I do think the overall boss fight was well designed and fun.