As I've said, I just go after EVERY trainer on my way and rarely use other than my starter. It levels so much it usually either one-hit kills, or can take out the other team quite easily. No matter the type, it usually holds up.
I'm still making my way to Black City from the top. I slowed down after the Elite four to find those legendaries, the sages, and all. Still need that last sage on Route 12, though.