I used treants from Force of Nauture talent, to make so that treants taunt the boss and i get the necrotic plague, because there need at-least 2 players to get necrotic plague. I was surprised when i saw that treant actually taunt the boss, because as i remember they can't taunt bosses.
So what i did was i summoned treant, got the necrotic plague on myself, waited till it ends/jumps on adds and started to kite, so that i don't get the plague again, because if I was too near the add with necrotic plague on then there was possibility that i would get plague again and 20 stacks of necrotic plague could pretty much 1 shot, so i had to be really careful (adds had to be near each other so the plague can jump to another add). After phase 1 the only thing that had left to do was to beat enrage timer.
Remember for achievement you have to watch stacks on adds not on Lich King!
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Immediate Music - Lacrimosa
Two Steps From Hell - Blackheart
Two Steps from Hell - El Dorado
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