Route 216 or 217
Mt coronet has higher level Pokemon, but there is nowhere to heal your Pokemon infinitely without having to go back down the whole mountain. You can use an escape rope, but to get the higher level Pokemon you need to go all the way back up the mountain.
At the end of route 216 there is a house, go inside and you can heal your Pokemon. Conveniently, there is a patch of grass right next to the house. Route 217 has a lot of trainers, which if you use the vs seeker on them you can grind on them, and when you get low on health go back to the house in route 216.
Hope I helped.