Hello,
I have not seen this posted anywhere yet and on the Cataclysm Dungeon Achievement Guide thread it said this achievement is very hard, but I do not believe that to be the case.
All you need is a party member who has a simple debuff. I suppose a DoT would work also but I have never tried it. I have done this with multiple groups as a druid and it has never failed. Here is how to do it:
1) Have the tank approach the middle group of mobs in front of the boss (there are 3 groups of 20).
2) When the tank engages this group of mobs, have someone (I'll use a druid as example) cast a debuff on the boss. I used Faerie Fire Feral.
3) By aggroing the middle group of mobs, all 3 should be aggroed to the tank at this point. Have the tank start walking them back slighty.
4) A void zone will spawn right behind where the middle group was. Have the group AoE these mobs as they stand in the void zone. They should all die and the achievement will be completed.
5) Kill boss as usual.
This may seem like common sense but I have been the only one in any group I've been in to mention it. I've done this 5 or so times without fail.
Good luck