This fight is divided into 3 phases. Phase 1 and Phase 2 are quite easy to deal with, with adds being easily nuked down in p1 and boss does minimal damage in phase 2. The only thing to watch out for in phase 2 is mana detonation. Hence SW:D should be used often.
The hard part of this fight is in phase 3, which requires a massive burn, and this makes this fight extremely hard. From a Shadow Priest standpoint, the adds must be tanked through the burn phase, and they hurt a lot. Overall From the time phase 3 starts, the boss must be burned down within 90 seconds or the adds would become too overwhelming. Like all burn phases, there should be no healing unless absolutely necessary. The boss begins to call for the adds when he is on 45% health, that means 2353k health must be burned down in 90 seconds.