"Together, we can stop him!" she calls to Sans. It's not the best greeting, but she's sure that's why he's here, too. A common cause should make this (relatively) easier.

And then she's in the crosshairs.

Sans may be nimble and a great dodger, but Toriel is not at all made for speed. She is, however, particularly sturdy. There's just the slightest shock at seeing Croc drop to all fours before she shifts her footing to brace for impact. It will hurt, that's for sure, but she can take it. Boss monsters are built tough.

Even as she's bracing herself, though, she calls on her fire magic again. Swirling embers spark into life around her like a fiery whirlwind. If Croc is going to hit her, it's going to hurt him, too.