I'm surprised no one's said Ceaseless Discharge. You literally kill him in 6 hits.
He's actually pretty challenging when you don't fight him like a b****.
You can still just fight him near the altar to stop him from using his OP moves like the fireblast thing, block/dodge the tentacle slam which is the only move he'll use, then go blow for blow till he's dead. You can even heal between as needed. Still an easy boss IMO.
In the midst of the word he was trying to say In the midst of his laughter and glee He had softly and suddenly vanished away For the Snark was a Boojum you see