Fanaelialae
Legend
Imagine you're the DM and a combat begins. You're running a bunch of bandits who, although they aren't the brightest sparks, have decent tactics and have ambushed more than a few travellers in their time.
A cleric in the PC's party immediately hails his god and calls forth divine powers to bolster the party. You think, "Ahah! Target priority number one!" And then this douchebag in heavy plate trips your bandit up and kicks him in the face whilst he's down. Another one of your bandits goes for the cleric only to have his shield sundered in half by this increasingly annoying plate-wearer, who then carves a big chunk out of the bandit to add injury to insult. And then the final straw, your BBEG goes to carve the cleric in two only to have his sword flung fifteen feet away by that darn plate wearer, and then be smashed in the face by a gauntleted fist.
Who is now the priority number one target on the battlefield?
You said that these bandits aren't complete idiots when it comes to battle, so why don't they just walk around the fighter (outside his reach)? After the first bandit sees how effective the fighter is, they ought to be scrambling to find themselves an easier target.
It seems to me that the best way to handle your proposed fighter is to never get into melee with him. After all, despite his effectiveness which might make him a priority, his armor will still make him an undesirable target (because he's harder to hit than the wizard). Thus, the best approach is to simply limit his control by giving him only one target to control.