Weak, yes. It's ironic that the term you use to describe a fire-based creature is frozen. It's the last thing you'd expect to see -- lava frozen within ice. By the description it seems more like a "brain and nervous system freeze," rather than encased by ice through and through (frozen). That or as I said, Chronomancer Timestop, which is something that makes a bit more sense as that's strong enough a spell to work on darn near anything save a deity.