An area with grass Pokemon would be best.
elite 4
elite 4
Route 201 or 202 or 203
i trained mine at mt.cornet
the place where you can breed Pokemon next to heart-home city, that's where i go
the best place to train is on snover in snowypoint
route 205 for Pokemon diamond, pearl and platinum. the trainers have Pokemon from level 10 to level 17
In the lake in Sunyshore City where Jasmine gives you hm7 waterfall.
find a place that has wild Pokemon the same level as it.
ANYWHERE
Eterna Forest.
sky piller