There are two ways to train up your weak Pokemon.
The first is rebattling trainers. Using the MapNav, you are able to see where all the trainers who are ready to be rebattled, and you can fly to said spot (You can fly to routes now) and battle said trainer. Alternatively, the Elite Four + Champion is also an option.
The second way, although you can only do it once a day, I find it really effective. This is dubbed as Blissey Secret Bases. There are QR codes on the Internet enabling you to scan them and the main Trainer (Ignore the grunts) wield 3 Blisseys, with only Healing Wish as an attack. So they effectively kill themselves and net you a ton of experience points. To add on, do activate the Level Release before fighting the trainer, as it removes level restrictions and make the Blisseys Level 100.
Here is a link to a helpful reddit thread that is trying to compile QR codes to these Blissey bases. Hope I helped!