The art style and music is stunning. I love the boss designs!
It would be nice if there's more anticipation in bosses' animation before they're doing their attacks so we as player can be more prepared of what comes next rather than just making guess on what they're going to do. The only time I can properly tell what's coming is when the second boss about to roll around. The rest of the bosses move barely have timeframe to react on what's coming. Even if we are aware of what the current pattern is, it's a bit hard to tell when the boss is going to execute the attack with the warning being constant or not present. It's easier if the warning is something is presented in the animation itself, just around 1-2 frames animation right before the attack execution.
Stellar art by the way, loving the background and everything else. I also like how the tutorial is presented. Good job.