reflection with a willpower check
I won't go there, sorry. Try breaking the spheres. You can throw stones at them. Maybe they need to be destroyed anyway, and then the boss will lose the reflection. I'll stand near the door, just in case – I'll launch a paper origami airplane at you. If you can't break them, then I'll see what to do next, maybe I'll go in. If you suddenly die trying to do that (it would be strange, but anything is possible), then unlucky.