This is a story related achievement and can't be missed if progressing through Champion mode.
Load up Champion Mode and you'll be straight into a scene where you've just been knocked down by a headbutt. Get back to your feet and Bring up your guard and avoid the opponent until the end of the round.
You'll be told in your corner that you have to knock him out in the next round. Simply go out swinging with combinations and he'll go down pretty easy. Try starting with an Uppercut then a barrage of left and right hooks.
He'll be KO'd and the achievement will be yours.
Easy