htf am i supposed to beat rafael

ALL GOOD NOW :)

OKAY I am on Explorer mode and have rerun this battle like 15 times and I CANNOT beat Raf. The stuff everyone has said to try is not working for me - he just gets out of Tasha's in one move (max three), my game says he is NOT polymorphable, my people keep dying via either fire or poison despite being all healed up at the start of the battle, and I don't know what destroying the pillars did - he still has 666 hp and my dudes cannot get that down past about 450 before they run out of spell slots and he kills them with hellfire. I can't use anything radiant because he has that stupid punish divinity thing, but they all seems to be immune to everything else? I have Will, Karlach, Shadowheart, Hope, and my draconian sorcerer tav in an effort to max healing abilities and fire resistance (which is not immunity especially not to hellfire).

This is my first time playing like any game and it has taken me almost 250 hours to get this far, idk what i did wrong that my peeps cannot do this for me.

EDIT: i'm level 12 and none of my ppl know hold monster, and i would need to roll a 30 to convince yurgir to join me (i think bc he hates me)

EDIT 2: I did it! Thanks - Shadowheart using feast and a djinni and using Hope's divine intervention mid-battle were the key. I was eventually able to corner him between Wyll's hunger of hadar and my tav's cloud of daggers while the rest of them just whaled on him.