This is making me want to go back and try again to beat Hollow Knight (I got up to the Mantis Lords and that... teleporting warlock guy... looked it up, I'm thinking of Soul Master.) I'm pretty sure I remember things of this sort of difficulty level in it, too. The fights were brutally hard, but still felt fair.

Mantis lords is a fantastic boss fight, because it's really hard, but super fair.

There's an attack during which you can consistently heal, so once you recognize that it just becomes about mastering the reactions to each of their attacks faster and faster and faster as the fight goes on. It's sooo satisfying.