Seems like the only way to get a solo duel is to wait towards the end of a dead instance. A friend and I had to spawn, and beat the Vigil for the Lost with just the two of us in the last 30m of the instance. But the duel was guaranteed to be one of ours! Was worth it imo. But ngl, this duel system is wack. I don't understand why you can't just pay Bozja crystals to do the fight. :/