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This achievement is completed on the Darkmaster Gandling fight, the last boss of the instance. 

Get the boss about about half dead, and kill off the adds he summons at the start with Rise!.  At that point, stop damage on him, because this achievement will take awhile.

Every 45 seconds, Gandling will teleport a player into one of the side rooms.  Unlike the original Scholomance versions, these doors open after a set amount of time and not by killing the NPCs inside.  DO NOT KILL THE ADDS.  Instead just do your best to survive until the door opens.  When your door opens, begin kiting the adds around the upstairs while the boss is tanked downstairs (so the tank does not pull aggro on them with any AoE or cleave effects).  Eventually the healer will have aggro on all of the adds, and will be the one responsible for kiting, and everyone else can chill downstairs by the boss.  Use any stuns and snares you can, just making sure not to damage them all.  You can also stand on the railing and jump off, to make them path back around.

After everyone has been teleported in once, there should be 50 adds being kited.  Collect them up and bring them down the stairs.  Snaring or stunning them will help keep them bunched.  At that point, it just a matter of AoEing them all down quickly and killing the boss!