I obtained the rank of Duke rather quickly but I can't seem to get to the next level.
What are the requirements for obtaining Grand Duke/Duchess?
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Sign up to join this communityAccording to Bulbapedia your Title is determined by how many battles you've already won. An exception exists for the title of the Grand Duke/Dutchess for which you have to defeat all Battle Chateau Trainers, excluding Gym Leaders, the Elite Four, and Diantha.
According to the German version of Bulbapedia the titles are obtained at:
You have to defeat every trainer in Battle Chateau at least once.
I didnt get promote to grand duke when I beat all trainer(I kept track of all trainers listed on bulbapedia). I quess you have to also beat certain amount of trainers to get the promotion.