Baron/Baronness - Beginning Title - Max. Trainers: 6 - Base Opponent Level: 15
Viscount/Viscountess - Defeat 5 Trainers to obtain - Max. Trainers: 7 - Base Opponent Level: 20
Earl/Countess - Defeat 20 Trainers to obtain - Max. Trainers: 8 - Base Opponent Level: 25
Marquis/Marchioness - Defeat 40 Trainers to obtain - Max. Trainers: 9 - Base Opponent Level: 35
Duke/Duchess - Defeat 140 Trainers to obtain - Max. Trainers: 10 - Base Opponent Level: 45
Grand Duke/Grand Duchess - Defeat all trainers in the Chateau to obtain - Max. Trainers: 10
Base Opponent level is the level the Pokemon is. For example, Xerosic's base Pokemon level (he doesn't appear in the Chateau, just an example) is 46.
Special Trainers:
Gym Leaders - Level 40 Pokemon as Marquis/Marchioness, Level 50 Pokemon as Duke/Duchess and higher
Elite Four - Level 55 Pokemon as Duke/Duchess and higher
Diantha - Level 60 Pokemon as Grand Duke/Grand Duchess
You can also write additional writs to increase the levels of Pokemon you face. These are:
Blue Writ of Challenge - Pokemon's levels are raised by -5
Writ of Challenge - Pokemon's levels are raised by 5
Red Writ of Challenge - Pokemon's levels are raised by 10
Black Writ of Challenge - Pokemon's levels are raised by 20
Source and Pokemon Gym Leaders, Elite Four and Diantha use here
