Not letting me upload for some reason but , "Coagulation of Flesh and Evil" his ability "Devour" is doing focus damage for 88.650 72.500 72.447 45.386 keeps going but one shotting. AOE blockers aren't even helping.
IIRC, you can fight him until the spawns the add - then drag him into the center green flame dome. Otherwise he will one shot you, I have no idea about the timer or if this script still works, I've killed it before its done its script on the last few TLE's.
You're getting a fail AOE. Fight him in his bubble and keep curing. Pretty much any named fight in TSO, if you get a massive AOE you can't joust, block, or mitigate, you've triggered one of possibly several fail conditions.