Scholomance

You need the Skeleton Key to open the door to the instance. If you do not have it, you can either die and resurrect behind the door, or complete the key quest chain at the Bulwarks if you are playing Horde.

Before entering, speak to the female ghost just outside the Scholomance entrance to pick up a quest chain, which also contributes to Loremaster of Eastern Kingdoms.

After entering, kill the mobs at the start before advancing — charging forward causes additional mobs above to jump down and aggro, which can be lethal even at high levels.

Head down the stairs to the left, unlock the Iron Gate, and enter the Chamber of Summoning, then run through to the door on the West side and continue forward into the room containing Plagued Hatchlings.

Run through the Plagued Hatchlings room and down the stairs to find Rattlegore on a hill — defeat him and loot his Viewing Room Key, which is required to access the room containing Ras Frostwhisper.

Return upstairs and work through the dungeon, defeating all six mini bosses — three upstairs and three downstairs — in the Headmaster's Study area, and clear all mobs in The Coven; Darkmaster Gandling will not appear until all of this is done.

Once all six mini bosses are dead and The Coven is cleared, Darkmaster Gandling will appear — defeat him to complete the achievement.