The goal of this achievement is to headbutt another player hard enough to knock them down. You do this by running with
, jumping with the same button, and then holding
This took a REALLY long time for me to get right, but after watching some videos on it, I think I have a better solution. If, like me, you keep head-butting your dummy/friend and it doesn't seen to be working, try to headbutt them in the back of the neck
For whatever reason, this worked the very first time for me. I was using a dummy in a local game at the time. Any map will do, though maps like Incinerator are best because they have long stretches of flat land. I'd recommend either Gang or Melee for the game mode.
Hope this helps!