User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-16281343-20150718214330/@comment-16281343-20150719013632

Think of it like these.

Ophis is really powerful. Nidhoggr? Not so much.

But the problem is Nidhoggr has a hostage. That doesn't just make Ophis unable to attack him, it also prevents her from defending as if Nidhoggr notices that he doesn't inflict damage, he will kill Issei's parents.

So, no attack and no defense. Even if you are really powerful, if you can't use them, then it's pointless.