This achievement can only be done in PRIDE MODE, kicks to a downed opponent are not allowed in the UFC. To enter pride mode select exhibition from the main menu and you will be given the choice of PRIDE or UFC. When your opponent is on their hands and knees, and you are standing up, hold and press or to perform a soccer kick. The first kick should Knock them out if they are already damaged.
Or knock down or take-down your opponent and go north south, throw knees, when your opponent is dazed stand up by clicking and hold and press or to soccer kick.
This can be done with a second controller, simply fail several takedowns and fall just in front of your first controller and soccer kick every time this might take around five kicks to knock them out.