Awesome game! The art is super charming, and it's very fun. Having different weapons to try, being able to choose the boss order, AND having upgrades between bosses gives the game a lot of replay value despite how short it is, after beating it the first time I felt inclined to go back and beat it with the other 2 weapons in different boss orders.
One minor issue I ran into was that if you're still holding left mouse after you defeat a boss it'll instantly select one of the upgrades before I could even see them.
Otherwise though I had a great time all the way through, really nice work :)