It's pretty much three games packed into one.
The first boss is cheeseable by just staying right in front of him, his slash attack and laser attack don't hit directly in front of him. The player hurt animation play after the slash animation if the player gets hit during the slash animation.
The second boss felt frustrating to fight. Since you can only attack after a long cooldown and if you're going to have penalty for leaving the screen, please don't use attacks that eat almost the entirety of your screen. (Still winable though) Music is a bit too chill in my opinion.
The third boss is... I don't know. I didn't get to see the third brother. Felt the invisibility frame was too short for the dash. Why is it so blurry?
There is no sound effect when you fight the bosses. It would be great to at least have hit feedback sound effect.
Edit : Forgot to write about the practice mode. It's really good to have that in the game, so everyone can at least experience every boss. Great addition.
Good job on the game.