Consider yourself lucky. On my last attempt, she didn't even show up. I must have scared her senseless.
She is there, you just need to use some AoE spell that will break her concentration ...
Just a little hint, not sure if that is intentional, or bug (more like bug) but when you cast AoE spell, and will move that spell abowe invisible target, you shall see its character target ring.
(sadly i dont have any screenshots atm)
