Defeat Elder Nadox in Ahn'kahet on Heroic Difficulty without killing any Ahn'kahar Guardians.
Everything in this post assumes you are level 90.
It is possible to solo this achievement if you have very high burst damage. I did it today on my level 90 ilvl 510 Fire-spec troll mage (25,906 spellpower) on my second attempt. (image) The important major glyph I used was Glyph of Combustion . My spell armor was Molten Armor .
For haste, I ate Tangy Yogurt and placed Rune of Power under my position. I then used Kafa Press , Berserking , and Time Warp . Mirror Images can benefit from Bloodlust-type spells, so I cast that spell between Berserking and Time Warp. I decided to not apply Living Bomb because I couldn't afford to use up a GCD (global cool-down) on a spell that did not do immediate damage. I cast Fireball a couple of times, cast Inferno Blast to help proc Pyroblast! cast the free Pyroblast, and cast Combustion . After that, I spammed Scorch for all I was worth because the offensive spell has a fast cast timer.
If I had been thinking, I would have changed up my talent tree so that I had Presence of Mind instead of Blazing Speed.
You cannot game the guardians by pulling Nadox to the dungeon entrance. The boss was fixed so that he beserks (500% bonus to damage) if he leaves his room. The Ahn'kahar Guardians were also given a sprint (200% bonus to movement) ability so they catch up. The guardians were changed to be immune to CC.
If you do not want to try soloing Elder Nadox, you will only need one other player. The two of you will stand on either end of the room, one person fighting Elder Nadox and the other tanking the Ahn'kahar Guardian(s). The key is to keep the boss a far enough distance away that the immunity buff will fall off after a bit. Whomever is tanking the guardians may want to remove weapons and some armor so there is no accidental slaying of the guardians.