IE: The backdoor to the endgame, bypassing the 3 effigies.

I think it has 4000 hp; and you don't benefit from Extra turns.

I thought maybe 4 Pipe Bombs would do it, but they a don't do it. Dansing or Adrenaline is out

I think the only option remaining is Spice; Double Orb or First Scorch+Double Combustion (And possibly party members using Lantern Flail, Ornamental Lamps for sure)

Not sure if First Flesh Puppetry will work, since I don't know how its timing interacts with doors. - there's also the Black Orb bug where party members gain black orb when spiced, which even without chac chacs should exceed their physical damage in most cases.