sky piller
find a place that has wild Pokemon the same level as it.
elite 4
ANYWHERE
An area with grass Pokemon would be best.
probly at the pokemon leuge
Mt. Silver or the Pokemon league
At the Pokemon league.
Some people will argue with me but i think the best place to train is at the Pokemon lugue you can battle them over and over again
victory road is a pretty good place
battle the elite four
victory road
grass