Strategy, for fistfighting? I won all of my fights like this:
Biff. Biff. Biff. Biff. Biff. Biff. Biff. Biff. Krunch. Thud. Victory dance.
Seriously: watch the animation. Your opponents will telegraph their blows every time. Click as soon as their hit animation starts, and you'll bop them every time. Don't even bother blocking, except if you get hit and thrown off balance; then it helps to regain your footing. The timing gets tighter as your opponents get stronger, but they're never particularly difficult.