just curious if this discussion ever went anywhere offline? If not, allow me to cast resurrection on the thread and offer my thoughts.
I'd leave aside magic, though that's a good option as well, and focus on weapons. Based on what I saw at the November event, the game has a decent focus on particular weapons doing specific things - two hander vs one hander vs axe or spear. With that in mind, make shield breaking the purview of two handed smash/bash weapons, specifically axes and hammers. Someone with such a weapon, using two hands, could deliver a strike to the shield, and three such strikes breaks the shield.
The need for a specific weapon and hand grip allow balance before it would be hard to get into range without help to deliver all three blows without the shield fighter just gutting you on their gladius.
Again, just my thoughts, a year or so late :D