Well, there's the Battle Resort, which has some high-leveled trainers you can fight once per day, some with slightly revolving teams. And there's the Battle Maison, which is, sadly, the exact same that it is in XY. And after you beat one of the Super Mode Chatelaines, you can rematch Wally with a stronger team. And the Elite Four have Mega Evolutions and are over Level 70.
Finally, there are a ton of legendary Pokémon to get your hands on, but some of them require unusual circumstances (check Serebii for more). Also, some of the trainers on the Trainers' Eyes will have much stronger Pokémon to gain EXP off of.