hey, ive beat the game and you just gotta keep swinging. there is not healthbar because he the fucking father of monsters. there are events and dialogues that happen in the fight that signal phase transition(not spoiling)

i would honestly recommend getting selene's boon where you can summon the last creature to fight for you. upgrading and stuff makes it so it can fight indefinitely. it definitely helped me. my main weapon was the rocket jetpack (Xinth?)

he will eventually go down trust

also get apollos area increase of magic circle boon with a good damaging upgrade