I lost to the Sinnoh league champion a few times because my Pokemon are about 10 levels too short. Where is a good place to train? { I beat all the gyms so I can't go there }! So where should I go that's in the area's can be in before the hall of fame?
The trainers on Victory Road and the Pokemon there should help. You can also stock up berries and heal your Pokemon that way so your money doesn't have to go into Hyper Potions and Revives.
the victory road it will take you allot of time but you'lll eventually get there but here's how you can do it use the vs thingy you use to rebattle other trainers in the period of time when it recharges you can just run around killing a bunch of wild Pokemon remember to pack elexirs for PP and some potions for the Hp just case.
Victory Road is the only option, and maybe the route before the last gym but in my opinion victory road is best. Also it is good if you have a looooottttt of revives and hyper potions etc... Getting past Cynthias garchomp will be tough .. :/