Kal is not killing Shaw with punches. Period. Even assuming Shaw has to mentally activate his abilities for them to work (I doubt he does), this is a duel. Both combatants are aware they are fighting at the beginning of a match and thus will have to think before their first move. Considering we have never seen Kal go from a dead stop to mach 100 in the blink of an eye it is absolutely foolish to believe he can punch Shaw in an arena setting before Shaw can think: "activate absorption." Kal cannot win this by punching or using heat vision.
That being said, Kal wins by clamping his hands over Shaw's mouth and nose and suffocating him.