^^ Not so sure about that.
Say a Skink tosses a javelin at a hero with the Charmed Shield. If the Skink rolls a 6 To-Hit, that's still a hit. It just happens to automatically wound. The Charmed Shield would still be able to discount that hit.
Also remember that you cannot pick and chose when to use the Charmed Shield. It always activates on the first hit. A smart opponent would knock it off with something insignificant like a block of archers, then follow up with a Cannon shot for the kill. Assuming he knows you have the Charmed Shield that is.