WoWDB

Not sure he could be soloed even if he was all by his lonesome. I can't recall the exact amount, but he had around 39mil HP.

Archiereus of Flame has the abilities of both Eternal Kilnmaster and Blazebound Chanter (except for  Fiery Charge and  Greater Pyroblast).

 Blazing Blow: This ability requires quick mobility, especially if Archiereus is augmented by the Eternal Kiln (more below). Depending on how well geared your character is, it will not one-shot you but still hurt very much.

 Cauterize: Interruptible. Although in the long term it is a negative return, it's still advised to prevent this yaungol from casting it.

 Conjure Eternal Kiln: The most threatening ability aside from the Flarecore Golem. When Eternal Kiln is summoned, Archiereus gains a buff from it called  Kilnfire. It increases his speed - movement, attack, and casting - and damage by 50%, which will prove fatal if the summoned mob is not destroyed immediately.

 Conjure Flarecore Golem: Summons a Flarecore Golem with ~39mil HP that hits like a truck. More than one can be active for the duration of the fight, and it will despawn upon the death of the summoner. Must be kited.

 Fire Storm: This ability shoots out swirly stuff in random directions. It will not one-shot a person (unless modified by Kilnfire), but be nice to your healers - avoid standing where the swirly projectiles will land.

{{coords here for my reference, ignore it: 50.1, 22.6}}