yeah, so the demon king being one-shottable is actually a bug where he is supposed to have 100 health, but when instantiating from the resource, it tries to set his current hp before updating his max hp, so it gets capped at the default max hp value of 18. But it's kind of good this bug shipped, because of the other bug you mention, which is that the potion button doesn't properly update its disable status in certain situations.
So yeah with even another hour i would have fixed those and also lowered the nick sprite spawn rate. I'm really happy with what I managed to get done, and I'm glad you liked the art, I had fun doing it. Really wish I'd gotten to draw the Demon King (the sprite that's in game was going to be the other normal enemy, but I didn't even get the boss in game until an hour before the submission cutoff)
Thanks for playing!