The visuals look great, I love the MS-paint art style. The concept is cool and some of the bosses look awesome. Unfortunately, it's really hard to avoid getting hit. It seems like the hitboxes are somewhat inaccurate and the very slow movement speed makes it hard to evade. I would suggest increasing the speed, adding some more range to the weapons and making it quicker to rematch bosses.
There's a lot of potential, I think this game would be really cool with a few tweaks!