He sees the swing -and the knife- coming, but he can't afford to dodge. Can't afford to give her that moment to get back on her feet, lest this fight drag out even longer. So he takes the hit, starts to go down with her.

And then, he summons up every dreg of magicka he's managed to recover and summons up his familiar one more time. The wolf tackles Natasha, and despite its ethereal appearance it isn't actually much lighter than a flesh-and-blood wolf of the same size would be.

That means the knife tears out of flesh and armor rather more violently than it went in, and he lets out a pained hiss, one hand reflexively covering the wound.