Mass-CC targets one creature at a time for sure. If you want proof, look at Vulture outgrowing a Rain of Fire. One Vulture dies, the others get +1|+1, then the next Vulture gets hit.
I can't say I like the idea of a creature with a natural attack this high at all, and it doesn't really fit Water either. In a lot of cases, Congealing or BB'ing something means it's almost as good as useless. Dying won't mean much of a difference when it does this much damage, really.