I really liked how the more you picked up the hearts, the bigger your hitbox got, but I kept wishing that with the bigger hit-box, it would have provided an extra ability to compensate (I kind of liked the idea of a dash, which I think could have been helpful).
I liked the boss but it was super difficult for me to win (though even though I kept getting hit when the boss was on I think its 3rd or 4th phase, it eventually said I overcame my fears, so maybe I outlasted it?)
Definitely a fun game to play. I would recommend taking a look at godot themes and some of the various textbox/label options to spruce up your tutorial page or ping us on discord if you want some help with that stuff.