Wow! Thank you for all this feedback, this is all gonna be really useful!
I agree that the boss is quite difficult, I think I would tone their abilities down a little if I had to make a change on the game.
So here's a strategy in case you want to see the rest of the game: Try and keep your soul as low as possible so that the boss' "appropriate" doesn't end up giving them too many soul, but not so low so that their "intimidate" outright kills you. It's a delicate balancing act that is difficult to maintain while damaging the boss if you don't pick the right abilities however. So even with this strategy, they end up being a little too difficult I think.
Anyway, I'm really glad you liked the game! :D