User blog comment:B214/Question and Answers 6/@comment-26926813-20161013185541/@comment-9595671-20161013200719

It's quite simple actually, Ophis doesn't really know about self-defense because of how long she was so powerful for. While she did shrug off a stab from the True Longinus that was just a single strike.

We have to consider how long Nidhoggr attacked her for and how much she let him do so to protect the egg and Issei's parents, we also don't really know how much damage her strikes did when we account he had Sephiroth Graal enhancements too.

Plus I think due to his self-resurrection ability, Nidhoggr doesn't have the best defenses. Like it's the result of gaining the ability or a side effect of simply having it.