Prioritize B, taunt A from ranged.
Prioritize A, taunt B from ranged.
Run over to the Adherent and try to grab aggro, dragging the Fanatic with you and risking cleave damage to all the melee DPS on the Adherent.
Throw a ranged taunt and just be ready for the next Fanatic. Cult fanatics can become sacrificed into a new add, such as a Reanimated Fanatic, or Deformed Fanatic.
B. And, if no one is on A, keep aggro on that too, so that the Deathchill Blasts from the Reanimated Adherent hits you only.
Tank A and B together, in melee range.
Taunt A, and kite B away until it's dead while maintaining aggro on B.
A, then B, then C.
Focus on B, range-taunt C then A.
Aggro B, kite A. Ignore C.
Aggro C, kite B. Ignore A.
Aggro A, kite B. Ignore C.
Start building aggro now so when Phase 2 starts, you can get aggro.
Start hitting the boss in melee range. When Phase 2 begins with an aggro wipe, immediately grab aggro.
Call out for the other tank to taunt off. Each stack is a 20% armor debuff and lasts for 30 sec.
Keep tanking until you get 5 stacks; it's a 20% DPS boost and helps your aggro.
Call out for the other tank to taunt off. Each stack is a 20% threat generation reduction and lasts for 30 sec.
Attempt to move the boss out, but most likely just get healed through it.
It's a 15-yard AOE of 12,000 dmg, run away! After the cast is finished, it becomes a Reanimated Fanatic or Deformed Fanatic (which you must kite).
You like being exploded, so stay in and wait for the cast to finish.