Thanks so much for all the feedback and compliments! I agree the big attacks the boss does should've been telegraphed differently, but basically it was meant to be an insta kill, but felt that was too extreme for a demo, so it's technically survivable at high HP. Spoilers ahead, but the proper way to deal with it is to make sure you kill at least one minion. He can summon up to three minions and when there are 3 minions on screen (alive or dead) his next special is always the ult. Any dead minions will be surrounded by a shield aura thing during that time which provides health Regen and immunity from the ultimate attack :)
Also, I really appreciate the art feedback! I'm an artist by necessity since I typically can't afford to pay someone for professional art, but learning pixel art has turned out to be a really great experience and I love hearing that my silly little style of characters is appreciated!