Steven lets go of the wrist--which he hadn't been planning to actually break, just cause pain to--and manages to move his head just enough that the headbutt lands on the side of his head and not straight on, where it'll actually break his nose.

"What did I say?" he growls, shoving Kirishima back from him. "Not in the kisser, kiddo."

This is probably a great time for him to grab another ball from his belt and let out his Klefki. (Meanwhile, Acatl the Xatu will try to use Thief on the Noibat.)