You need to kill Primordius without anyone absorbing a Mutagenic Pool. http://www.wowhead.com/achievement=8037
On 10 man Normal, boss has 218 mil health.
218 mil * 4 (75% damage reduction without Fully Mutated) / 480 seconds (8 minute Berserk) = 1.81 million raid DPS sustained for 8 minutes. This doesn't seem possible without T16 Heroic gear.