Dark Priest Xanarch
GrimTools:
https://www.grimtools.com/calc/bVA9q31V
This is a “Dark One” set healer build, with 2x wendigo totems, 2x heal birbs, and provides 30% physical resistance, over 400 offensive ability, about 600 HP regen, and 8% defensive ability to all nearby teammates, plus a number of other resists and benefits. Stone Form also provides 400 damage absorption from time to time. 
To showcase the sustain provided, here is a video facetanking Callagadra. It is not a fast fight by any means, this build is designed for team healing and sustain, not for damage dealing.
One thing critical about this build is pet positioning; keeping them alive will keep you and the team alive. That means, with certain fights like this one, there are certain spots where the pets must be just outside of the primary line of fire so they don’t die, and just close enough to the wendigo totems so that they benefit from the healing of them.
Enjoy
Video: Facetanking Callagadra