Antimony
From DDwiki

The boss of MOAR! is Antimony, who has a huge health pool and nearly-impenetrable
Physical resist. You will need to cast many
BURNDAYRAZs to take him down. A mid-fight level-up is of course valuable, but over the course of the fight the
Burning stack will become significant and you can squeeze out dozens or even hundreds of points of damage if you don't lose it by attacking some other monster. Taking
The Pactmaker's
Alchemist's Pact can provide you with the mid-fight level-up -- Take
Consensus first if you need piety to fuel it. Besides
Alchemist's Pact, you can use your potions, and hopefully you have saved some glyphs to use with
Refreshment.