headbutts

I wrenched him away and reared back my head and pulled him in and slammed my forehead into his face once then again and again screaming and roaring and raging until he went limp.

...

“So that’s what it was,” said Gow. “He had a hard skull. Obviously not as hard as yours. But congratulations, you fractured two of his vertebrae.”
Sledge vs. The Labyrinth, pg. 214-218