Edit: I just figured it out. I had to stand in the beam that Aurene makes once the Branded Aurene is stunned. My mistake was thinking that a white circle would appear on the ground like the rest of our attacks. Oops! Sorry for the worthless post now haha.
I was wondering if anyone knew what the conditions were for being able to do the special attack that is mentioned in the achievement. I've tried not breaking Branded Aurene's breakbar, tried breaking it as fast as possible and have done the fight a few times now. I've kept my eyes to the ground during the fight to see if a white circle would appear but haven't seen anything.
Can anyone help me to figure out what I'm supposed to do to trigger the ability to do the special attack?