Wow, I expect it to be longer because it's so fun! I love the different weapon mechanism that get upgraded using the previous boss materials. Even though the boss is going insane the lower health it get, it's still readable.
My suggestion is the movement is could be less slippery and the player gravity is too high. Some transition of this game is still a bit too long, like the auto restart when the player loses.
Overall, it's a pretty good fun game, great job!