Usually a quick, hard punch to the belly, followed by a hard slam to the nose will get the job done. Most of them you can surprise with a sudden uppercut to the chin. Then bring the left to the belly. When he brings his arms down, pop him on the nose. The nose is the place that a hard lick can cause the most damage and end the fight.
Nothing hurts more than a hard lick on the nose.
Hit it on the side. This tears it up more. He told me that most inexperience fighters just swing wildly, usually hitting nothing more than your arms or the side of your head. When an opponent starts swinging wildly, bring up the quick, hard uppercut. That was good advice and it worked well.