Oh theres plenty of quest duels you can stay asleep over .. probably right up to lvl 50 if your good at it... hell there even a medal or two to be gained I believe
Speaking of which, I myself have been getting these challenges from higher duelists. And I am not even a duelist, nor do i have the option to make the choice to be a duelist at this time. Remember those KO's when the ones that have done this boast about their "duels"