This game was one of the staples of my earlier life, though I haven't played since 2002 (when I was 38 and my uncle 51; it is good to stay in practice). Great game for getting used to being punched at, defending yourself and punching back. Not infrequently does the game evolve into an actual and for-real fistfight. God it was fun being young!!!
A lot, if not most participants, try to win by overwhelming the opposition with a volume of punches, and, at some level you've got to go with that, just to stay in the game. Keep your arms in close to protect yourself and use your elbows to push/parry/block shots. Punch inside his punches- shift the weight on your feet and torque your hips to drive your punches. (getting in close enough to literally keep your elbows at your hips as you punch should work ok. Most people are arm punchers and need room so take that room away) Also if you punchwith your body and not your arms you'll be punching longer and stronger, which is an advantage in any punching-oriented competition.
Of course, as this activity is probably not supervised and does not involve proper protective equipment, it should be avoided.