Kralin Thornberry
Explorer
Maybe the shield doesn't stack with armor (like old mage armow). That would seem to balance out with other defenders (actually a little lower?) except he wouldn't be armored. The damage reduction is pretty ridiculous, but maybe theres some detail that were missing that leads to a more balanced ability.
Actually, it is to a single attack only, so if the mark is used on a wizard that is using mass damage spells, it's only good for one of the attacks.