This is not a difficult achievement, but rather an annoying one. You need to kill Auriaya's Feral Defender 9 times in order to get credit. He first spawns 1 minute into the fight and then respawns 30-45 seconds after its death. Each time he dies, a void zone appears on the ground that does an immense amount of damage. Needless to say, you need to get out of them as quickly as possible. It helps to kill both packs of trash on each side of the platform so that you have room to move around. It's not advisable to attack Freya until you've killed the Feral Defender completely as you will probably kill her before you get a chance to finish his 9 lives.