Elite Four Grinding - Beat the Elite Four and Diantha, should yield enough XP for leveling up from Level 1 to Level 36.
Grand Duchess Diantha - In the Battle Chateau, once you are of the Grand Duke/Duchess rank, you can battle four of Diantha's Pokemon. If you issue a Black Writ of Challenge, they are all Level 80.
- Battle Chateau - It in itself is a great way to train your Pokemon. It has relativity high level Pokemon and is a good source for experience.
These are two good ways to train. There are plenty of other ways like smashing through Victory Road, but these are the two most efficient ways to train.