Croc, well enough used to seeing lions coaxed through flaming rings, knows the hierarchy of flame. Blue means stay away. Blue means scales cracked and smoking. Even without painful experience on his side, something deep in his (heh) lizard brain cries out in primal terror at the wall of flames. While he's able to ignore it, if need be, now is the time to listen.

He makes up his mind to let Sans lay there and bleed out, and doesn't notice the other skeleton run to his aid. Instead he keeps on his barreling track to Toriel.

When the fireball hits, Croc does a full flip backwards and crashes into a bookshelf from the force. He, himself, has a new burns spreading from the base of his throat until his waist, scales cracked and glistening from the blistering heat. He growls something out-- a curse, an obscenity-- and passes out.