If I recall correctly, this came up in an old thread. Ah, found it:
Brawling it is. In general, shields are effective because of the fighting style used, which means what weapon it’s paired with, so you always use the skill of your weapon to hit with a shield. When you’re armed with a sword, that’s Sword, and when unarmed, that’s Brawling, whether blocking or bashing with the shield.
I can’t see using Boxing, since a shield would interfere with the form.