This boss was actually pretty trivial for a priest. Phase 1 is just an add control phase. Basically adds should be killed fast enough so that the friendly npcs don't die. On the other hand it's also advised not to be overprotective of the npcs and grab all the aggro onto yourself since that means more healing required, draining more mana.
If there is enough npcs kept alive from phase 1, phase 2 is pretty easy. Simply fade and let the npcs do the tanking.