Get near the end of Act 5 on Co-op Campaign mode. Whenyou have just used the Troika Gun Turrets to defeat the Reavers, have a player climb down the ladder and walk to the nearby door, as if about to start the intermission sequence. Then have the other player climb down the ladder, but climb immediately back up as the other player enters the door to start the intermission sequence. This will lock the ladder and trap the player up the ladder with the turrets. The other player will not be able to progress any further or climb back up to join the other player.