Halls of Stone

Did this one with a tank warrior, a dps warrior, arcane mage, a paladin who's spec for healing, and a hunter, first boss had a somewhat final attack which took out both 80 warriors, second boss had the pillars which had the same effect as seen in Drak'Tharon Keep, the 'third boss' which really is just a group of adds just running in from the doorway (take out the 79s before attempting the quest thats in the room next to him) stay near the doorway as best as you can and you'll do fine, the last boss was tough you definatly need nature resist for him stay on him and he'll go down.

A fairly easy achievement in Cata and beyond - you should have no problem soloing it on an 85+ toon.
However, with the addition of shared achievements I simply leveled an alt and got this - it is simple; even if someone drops group, you can 4 man it (that also means more XP!).
It's a long dungeon, so just remind everyone that you can run it all and get the achieve and never come back, or, well...come back!