Really, you have no idea. Because after Levi's finished with him, mirror!Phedre - who is a sadist and extraordinarily good at causing people pain - is going to help patch him up. He won't know it's not really her, and he'll trust her and open up to her - making him incredibly more vulnerable for her attack. Which is going to start with just emotional abuse, undermining his self-esteem and his relationship with Sansa, and and he probably won't even realize she's doing it at first. And then the physical and possibly sexual abuse will come later.

For much the same reason, really - older Armin stops her from hurting Alcuin, fighting her and at one point holding her at bay with a sword to her throat. Sheee doesn't take that well, so her abuse of little Armin is going to be particularly cruel.