Battle Maison is so hard. First ten matches are easy but you get double BPs for a reason once your streak hits double digits. The opponents' Pokemon have this annoying tendency to "hang on" with just a sliver of HP and still land shots on your Pokemon, and since they use a variety of attacks, they'll probably find a mismatch. Also, they have Berries. I was fighting against a Bug-type Pokemon and given that Bugs have so many weaknesses, I was surprised to find one of the Battle Maison trainers had the EXACT Berry for the type of move I was planning to use and that dampened my super-effective move. If I didn't know any better, I would have thought that maybe the game just added that Berry at the last minute just to dampen the attack, right before I was going to use it. There is a diabolus ex machina you have to defeat if you want to reach 20 wins at the Battle Chateau.
I know that the only reward for fighting is more BP, but I still want to beat one of the Chatelains just to say that I did.