Mitsuya didn't do much but a small blunt-force impact to the creature-- like a shove, or a push. Which, combined with stairs only equals sad times.

"Wo-WOAH, WOAH!!!" the crocodile shouted as he fell down, rolling down the stairs, landing at the bottom with a particularly loud and almost cartoonish sound.

A moment into it, after some pained writhing, he got up on his feet, growling as he tried to start climbing back up. "Stupid short legs, ooohhh, stupid, stupid, stupid...!"