Volazj's Quick Demise

Having a melee-heavy group with a Protection Warrior tank is the key to breezing through this achievement with ease. Melee classes have the easiest time polishing off their shades, but the Protection Warrior is what really trivializes the DPS requirement. Spell Reflecting Volazj's Mind Flay will cause him to take ~18 000 damage for every Flay you send back at him.

Assuming you take 30 seconds each to handle the Insanity phases, and your Warrior reflects a Mind Flay every 10 seconds, your four non-healers need to put out only an average of 1348 DPS.

Edit: This post is now for historical information purposes only. This method was hotfixed as of 9th December 2008's Tuesday maintenance and no longer works.

(Don't downrate this or half the comments below won't make any sense about a LoS method!)

---------------
Tank the boss at either of the 2 pillars behind the boss and have everyone stand next to the pillar as well. When the boss casts Insanity (5 second cast), all players run around the pillar to line-of-sight the boss.

When the cast completes, assuming all 5 players were out of line of sight, everyone would briefly see 4 images of their party members for 1 second, and everyone exits the Insanity phase instantly (the images kill themselves). So Insanity pretty much does nothing, and you can kill the boss easily within 2 minutes.

Note you may need to LoS Insanity twice.

Did it tonight with Prot warrior, Ret Pally, Enh Shammy, Holy Pally, and myself(FG/ember lock). What we did was pop heroism instantly at the beginning and burned him to about 45% before the first insanity hit, and then used that same heroism to burn through the first insanity as fast as possible. We then pushed him down towards the second insanity, and as he was casting it, our Holy pally(who I had soulstoned) DI'd me. I broke DI once he had cast, and had no adds. Our pally rezzed and had no adds. The other three only had to deal with two adds each. Killed him with about 25 seconds left.