Temple of Ahn'Qiraj Guild Run

You need to be a minimum of 10 people from the same guild in order to be eligible for this achievement.

My original comment up top was about RAQ, not TAQ, so it got down-voted. Here's the corrected info:

Our guild achieved this on 1/16/11. This is the raid formerly known as AQ40. It's still a 40-person instance as of Cataclysm.

To earn the guild achievement, you must have 10 people in the raid be members of the guild. (You can bring up to 30 non-guilded raiders, as long as you have 10 guild members.)

You do not have to kill every boss. You can skip any of the optional bosses. The non-optional bosses are:
* The Prophet Skeram
* Battleguard Sartura
* Fankriss the Unyielding
* Princess Huhuran
* The Twin Emperors : Emperor Vek'lor & Emperor Vek'nilash
* C'Thun

You'll get some amount of guild XP for each boss kill, but only killing C'Thun grants this achievement.

The optional bosses are:
* Vem, Yauj, and Kri
* Viscidus -- very difficult if you don't have a lot of frost damage to freeze it, and then a lot of melee attacks to destroy it when frozen
* Ouro the Sandworm

For 85s, the place is mostly easy, but you can still die to falling damage from the mobs with knockback, and the Twin Emperors fight can be difficult if you don't know how the mechanic works (keep them separate, no AOE or dots, need 2 melee tanks and 2 spell casting tanks).

You can do this with about a half-dozen 85s.

BUT to get the achievement, you still need to have 10 in the instance, even if only a few are actually there to engage C'thun (the rest can be parked by the door).

I really love how they set up achievements for killing the vanilla bosses. Really wish they would make them more difficult though. Oh well, always fun to go back in the old raids. Still believe they're my favorites.