Mob is extremely easy until it reaches 10%. At 10% the mob will emote, "Forces of darkness, I call on you for the strength to prevail!"
It is at this time he will begin to cast a massive spell that if cast will wipe your raid. Two ways to prevent this are:
Coercer/Illusionist single target Stun about 2 seconds after the Emote will most likely prevent his casting of this ability for at least 20 seconds.
If no Coercer or illusionist in raid the only other option is to AOE Prevent every group in the raid.
At 10% BURN like there is no tomorrow, using the Heartwood stakes also helps a ton on this mob as you can use it when the mob hits 6% health.