Always seemed to have trouble with the 1st boss in this instance when a pali or a druid tanked him.
Simple answer folks. Pali's stop using holy sheild, druids stop using thorns. They activate his "shadow shell" for instant 2-3k hits. this hits regardless of regular attacks and as many times as thorns/holysheild procs against him.
Once we realized this, he became easy to heal through.
Just a lil fyi that'll make your tanks not need 5 billion shadow resist (our druid tank could then take him with only the priest shadow buff.)