Complete The Crumbling Hall and The Thunder Below without taking damage from any source.
Huge pain of an achievement.
The Thunder Below can be easily done with help. If you just walk into the first title room and stand there while a friend in your party goes to make the kill you'll get credit. I would suggest that you hearth out and fly back, so that nothing can hurt you on the way back. As for soloing it...I honestly have no suggestion here. I have read others ideas of how to solo this and I still tend to get hit when the boss pulls you down right into the sparkly floor.
The Crumbling Hall might require someone of the same faction if you wanted help with it. All the statues that come to life will do so when you enter melee range, so if you stay as far as you can they won't activate. This is harder than it sounds at times, like when there's two guarding an activating device or when you need to sneak past them in the last room. I (as Horde) saw an Alliance running in front of me and activated one that she ran right into. I'm not sure what this means for how much a friend can help. Either you need someone in the same party, or just the same faction to make sure that the statues are safe to run past. Once you have your device at the end once again, hearth and fly back.
When you turn in make sure you dismount. This expansion has had odd bugs with being mounted and not getting achievement credit.
Still obtainable in patch 5.4.