Anywhere that u find Pokemon that are close to the shromishes level.
elite 4
elite 4
the elite 4 or the places with the highest lvl wild Pokemon in them
An area with grass Pokemon would be best.
Route 201 or 202 or 203
the place where you can breed Pokemon next to heart-home city, that's where i go
i trained mine at mt.cornet
iron island or upper level mt coronet
Stark mountain battle about 7 Pokemon then it will be level 73.
You train them
route 205 for Pokemon diamond, pearl and platinum. the trainers have Pokemon from level 10 to level 17
the best place to train is on snover in snowypoint