She can flashfreeze and heat him up repeatedly until his body cracks, IIRC there was a fight in which pyro and avalanche try to do it with liquid nitrogen and pyrokinesis and I think it was stated that it would work.
Storm also claimed a steel spike accelerated by a hurricane could shatter frost's diamond form.
She coul also dump him in the ocean and let him sink, would that work? Can he float? If he cant float but can swim because of his strengh, she could manipualte the currents to take him to the floor of the ocean. Even if he swims up, it would take a very long time and should count as defeat by incapacitation, although it migh be seen as bfr.