Where's a good place to train up to level 50?
Honestly, Mount Coronet would probably be the best place for this. Entering from Route 216, the Pokemon in this area are in the low to mid 30s, and they get stronger as you go up, ending up at about level 40 on average at the top. The area around Lake Acuity has about the same levels as higher up on Mount Coronet, and then Route 222 has Pokemon in the low to mid 40s, which should help boost your team up to level 50.