In the 10 Player version of this fight, there are 6 Heroes to defeat. Below is a list of possible classes/specs you will face.

Class	Spec	Champion of the Alliance	Champion of the Horde
Death Knight	Frost	Tyrius Duskblade	Gorgrim Shadowcleave
Druid	Balance	Kavina Grovesong	Birana Stormhoof
Druid	Restoration	Melador Valestrider	Erin Misthoof
Hunter	Survival	Alyssia Moonstalker (has pet)	Ruj'kah (has pet)
Mage	Arcane	Noozle Whizzlestick	Ginselle Blightslinger
Paladin	Holy	Velanaa	Liandra Suncaller
Paladin	Retribution	Baelnor Lightbearer	Malithas Brightblade
Priest	Discipline	Anthar Forgemender	Caiphus the Stern
Priest	Shadow	Brienna Nightfell	Vivienne Blackwhisper
Rogue	Subtlety	Irieth Shadowstep	Maz'dinah
Shaman	Enhancement	Shaabad	Broln Stouthorn
Shaman	Restoration	Saamul	Thrakgar
Warlock	Affliction	Serissa Grimdabbler (has pet)	Harkzog (has pet)
Warrior	Arms	Shocuul	Narrhok Steelbreaker

The fight is much like a PVP encounter. Each NPC will use their abilities to the fullest (i.e. healing, bubbles, immunities, etc.). For this fight you don't really need a "tank", so have your designated one DPS and be on taunting duty. The Rogue will Shadowstep people, Shamans will use totems and Bloodlust/Heroism, Paladins will use immunity bubble and Mages will use Iceblock. It's best to bring along a Priest or Shaman who can dispel/purge.

In order to get all 6 down within 60 seconds of the first death, you'll need to coordinate stuns/fears/silences/CC. Try taking them down in groups of three. Focus one target within the three (priority being the healer) and go from there. Pets within the enemy group do not count towards the achievement, so it's a good idea to kill them in the beginning of the fight.