Solution posted originally at
http://www.rare.co.uk/blog/?p=2528Launch as many uppercuts as you can possibly manage and the opponent will have a hard time blocking, mainly because even when they block low, the upward motion of the punch will still mean they get punched in the face.
The opponent will almost always be stunned after a barrage of uppercuts, and all it takes is one more uppercut or a powerful cross from the side where you follow through with your shoulder. Your biggest enemy here will probably be fatigue, so revert to rope-a-dope style punches if you start to run out of steam near the end. See the video below for both Boxing Party Pack achievements.