It's time for him to change tactics as well. He's been leaning heavily on his magic, and his reserves of magicka are dwindling rapidly.

While she's down he lets go of the invisibility spell, but not Oakflesh quite yet; he does remember that she has more weapons in her arsenal than just the guns. Then he emerges from his hiding spot, rushing toward her. If they keep this to a ranged fight, he is eventually going to run out of magicka, and the utility of the crossbow bolts is limited since he doesn't actually want to kill her. So, close-quarters combat it is.

He fully expects that she's better than she seemed when they sparred, given that she's no longer operating with the handicap of being in the wrong body. Even if she is, though, he still has an advantage of reach and weight that can't be discounted.