So that series of attacks went off pretty well, but the Medusa had put itself on the defensive. Alright, they'll just have to be more careful with their strikes...

... and then the snakes begin rising up.

He probably shouldn't turn his back on the monster while he's so close, but it was faster for him than shutting his eyes and he could take the opportunity to breakaway from immediate combat to use a hi-potion on himself. Next turn - if Papyrus and Frisk's Melodies didn't finish it off - he'd be ready.