Doable in a New Game+, but I would certainly say it's the hardest fight in the game. Beating it on a first play through, now that’s something. (Maybe I'll try over Christmas when I'm away from my consoles, gogo frame skip leveling).

Even if you manage to level enough, the lack of the best armor is the real kicker.

(On a side note, if you beat Lavos form #1, and then gate out before the ocean palace, when he spawns he will also be headless. Kind of amusing having the big build up to terrible Lavos and he appears decapitated.)