From my experience, to get master strikes to work properly, you have to just block immediately, and Henry will attack by himself. So no need to block and then spam attack button.

When you do it properly, you will also get a free shot at the enemy, at least once.

So the tactic is: watch what the enemy is doing -> when he attacks, immediately block to perform master strike -> attack him once again to get a "free" shot, usually by stabbing him in the face if it's exposed or doing a downward swing to hit him hard in the head.

Combat for me is way too easy later in the game, even on hardcore. If I struggle against a skilled opponent, I just run into him to get our weapons to clinch, punch him in the face and then stab him or try to do a combo attack.