Speedbag

You quickly swing your fist, landing a right hook to his left orb. He lets out an agonized moan as his left ball recoils, bouncing obscenely.

This gives you an idea. You begin moving your fists like a boxer does to a speedbag, striking him right in his big balls. Each time you hit a ball, it bounces back into your other punch.

The zombie freezes in place, in too much pain and shock to move. Sweat begins to trickle down his forehead as you box away on his balls like a windmill.

With him right where you want him and your fists making thudding noises against his balls by the minute, you begin deciding your finishing move.