Defeat the Feral Defender while defeating Auriaya in 25-player mode.
Feral Defender has a buff woth 9 stacks called Feral Essence. Each time it revives it will remove one of the stacks. Kill the Defender 9 times and then kill Auriaya to gain the achievement.
Be aware that when killed the Feral Defender drops a void zone that does 9,000 shadow damage per second. While doing this achievement you'll need to continue moving around the room due to these void zones eating up the available tanking area.