Defend your head, watch his eyes and read his movements.

Keep as little as possible surface area exposed to him, assume a fighting stance using your least dominant arm shoulder as a guard.
Swing with your most dominant with speed and clenching your first before contact will make the hit count more. Immediately come back with the least dominant, and again with your most dominant.

I usually fight in hits of three, or a frenzie of punches at once giving no time to retaliate. If you hit them fast enough they will try to defend usually and leave parts open such as kidneys, balls and neck/face.

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"As long as there's free will, idiots will never learn"