It's probably a fun game.
I can't say much about gameplay as I'm kind of having skill issue with FPS or 3D shooting games which requires precise aim especially if my character have to constantly moving.
I don't know much about the difficulties but I'm pretty sure you need to give a signal for the boss attacks especially if it's instant long range attacks like that water beam, it can be like charging animation or warning signs so player can anticipated or maybe it's just me having a skill issue and couldn't read the timing perfectly, my bad then.
Again, I can't really say much about this game so that's all I guess, if I write something wrong or missed something, let me know.