The Orb Whisperer (10 player)

We got this one tonight, there are 3 things (each caused by the empowered boss) which could cause you to lose this achievement.

Valanar: Empowered Shock Vortex. When he casts it, you must spread out quickly to avoid multiple people blowing up near one another. I'm not too sure if for the purpose of this achievement each hit counts only as 7k damage, or if damage is cumulative, so this one may not be strictly necessary, however it's something you'll want to do to survive the fight, so ehh, might as well do it anyway.

Taldaram: Empowered Ball of Flames. When this is cast, the player who it focuses needs to run away. The further it has to move, the less damage it does (it's also believed that the more damage it does to other players as it moves the less damage it does on final impact, however we didn't focus on this, and simply running away to the back of the room was enough).

Keleseth: Empowered Shadow Lance. Your Keleseth tank wants to have 4 stacks of the Dark Nucleii debuff up at all times, so that when Keleseth becomes empowered he will deal significantly lower damage for the duration. This should be done regardless, as our healers had some trouble keeping our ranged tank (a warlock) up with 3 stacks, and found it literally impossible with 2.

You are more right than you think, check this quote:

IC_Valanar_Empower01 - Naxxanar was merely a setback!

So yeah

as others have said this one is all about the keleseth tank. (commonly a warlock)

if you're the keleseth tank get 2 or three orbs at all times when kel is not empowered and nuke to get a good threat lead (demo locks can soul fire throughout due to his health being 1hp while not empowered).

before each switch and during keleseth's empowerment just keep stacking orbs, even 5 or 6 is fine. your threat should be good from the other phases to maintain agro.

one tip is to look out for (and avoid) passive AoEs like consecration because it's very easy to have all your orbs dragged off you as you run to get more.

the orbs agro the last player to damage them, they don't have a true threat table. as such the best way to keep agro is to put a DoT on them so that if they do get damage from another source they will come back to you eventually. this mechanic is also why consecration is particularly dangerous.

note for warlocks tanking him:
demo builds (or destro builds which take soul link) are best so that damage is shared with your pet.
demo locks should use a felhunter for the spell damage reduction via master demonologist.
other locks should use a voidwalker for the bubble.
use demon armour instead of fel armour for the increased healing received.
use CoA to agro orbs. use shadowflame to maintain agro on the orbs you have (AoE DoT)
cast shadow ward as keleseth empowers (it won't absorb much but every little helps if you're going for this achievement)