Well three areas you want to focus all blows to, throat, groin, kneecaps. Hit him in the throat, he can't breathe, hit him in the groin, he goes down, kick him square on top of the knee cap, again he goes down. But the one I've done, especially when they're bigger is a direct head butt to the bridge of his nose followed by a swift upper cut to the groin, you can add a grabbing twisting motion to this to further intensify the pain, after this give him a knee to his face. I gaurantee when executed properly you'll walk away without a scratch and he'll be left laying on the ground with a mixture of blood and tears in his eyes. Of course while he's laying there it is customary to give him a few repeated blows to the face or midsection before exiting the area.
God I miss those days sometimes.
