well, the way I train depends on its moveset types. I've only experimented with a few types, though
Level 1 - 35ish All pokes : Poke league with exp share
Level 35 - 45 Grass, water, fighting, ground: Iron island is the next area for the types that can killl rock and steel types.
40 - 60 Fire, fighting, Rock. Acuity Lakefront. If you're training a rock poke, flee from Machoke and Medicham. by destroying Ice types here, 1-hit KO's and fast Xp are yours. With a fighting type, have it knowing a Ice or Electric move for hoot-hoots, Noctowls, and Medichams, as they'll ruin your 1-hit KO parade if you don't
50 - 65 Grass, water, fighting. Victory Road will do you some good here. Enter from the Mansion entrence, and have at it. Again, with a fighting type, have it knowing a Ice or Electric move for Medichams.
60+ Fire, fighting: Snowpoint temple. It's like Acuity Lakefront, but Golbats are your worry if you're fighting. Steelixes give a decent amount of experence points, so always kill them. I got my first shiny (a steelix) here, and a Lucario up to 100. In the later levels, you'll get a level about once every 25 minutes here.
55+ Grass, Electric. Resort Area Lake. With Golduck being the one and only pokemon here, ranging from level 38 to 55, your trainee should have fun killing Golducks. A Blissey with seismic toss got to level 100 at this location
60+ All pokes. Lead the team into the Elite Four! Switch out if there's an opponent that can kill you.