I just kept using the Vs. Seeker on Route 222 to train my Yanmega super hard. Started at 35, beat league at 65 ;3. Also, I highly recommend that you get a Sneasel/Weavile. It helped me with Lucian and Cynthia's Garchomp. And if you get Weavile, keep Icy Wind on it so you can slow down Cynthia's Garchomp so you can kill it easily.
Hope I helped!
I battled the Rich Boy, Lady, Cops, and the Fishermen to train my Pokemon btw.