This achievement can be easily done with two tanks, two healers and a DPS. The first tank and healer will try to keep King Dred alive long enough for the 2nd Tank, Healer and DPS to kill enough raptors.
When the fight starts, the Main tank and healer will pull King Dred. As the main tank, your job is to keep King Dred on you, not kill him, so try not to do too much damage and only keep him off the healer that's trying to keep you alive.
The other group will pull each raptor one by one and kill them as quickly as possible. Be aware that, since the raptors occasionally charge a random player, depending on where the other group is, it is possible for a raptor to become stuck in a wall. If this happens, you may need to wait for it to respawn. When the last raptor dies, rejoin the other group and finish the boss off.