Eiro covered up Ariella with his wings, trying to ensure that the crow couldn't get to her. Fighting that monster while protecting someone wasn't part of Eiro's plan, and there really wasn't anywhere for her to go right now. And the problem wasn't that Ariella was weak, of course; she wasn't weak at all, but even Eiro wasn't able to physically respond to the crow's movements. All he could do was use the domain of truth to predict their next actions as best as he could and doing what he needed to.
Right now, the crow was stood still. As though they were watching Eiro. He realized there was a chance.
