Power punches working the inside. Once you are close enough throw a hard uppercut to the body, aim for the very bottom of the rib cage in between the elbows. If you hit your opponent hard enough he should buckle up a bit, use your left to keep the correct distance by either collar tying him or using the elbow as a wedge and set up for the right uppercut to the chin. It isn't sophisticated or even skillful but it will knock them down and keep them down.