Dun does exactly one thing: hook someone, knock them up, and hope his team follows up. His entire value in a teamfight is that hook. If he lands it on your carry, you lose the fight. If he misses it or hooks the wrong target, he's a walking HP bar with no impact.
Countering Dun is about three things: dodging the hook, punishing the miss, and not letting him engage on your carries.