Ooo I see about the health thing. I guess it is a bit tricky. As each boss is vulnerable in different ways that means their HP holds a different difficulty value. Like Magic Hat is practically free to hit and Crazy Cat is harder and can hurt you by comparison. So increasing HP to some bosses is way harder and to others meaningless. I'd generally say it's better to focus more on other aspects of difficulty as well and let the bosses be short if you are good. Though if wanting to do HP increases, then I'd say each boss should have their own Base HP & HP Scaling.