There are three duels in the game two on the first level (western) and one on the second (train). A duel is when your characters stand in front of an enemy team and the screen goes dark. On the bottom there is a time limit which is not much on hard. You have to mark every enemy during this time on the screen. A crosshair moving randomly around on the battlefield, stopping for more or less time on the enemies (turning red). You have to press
to mark them then. If you miss once you have to replay the whole level. I got it for the first time so I don't know if you have to do them in order or not.