Hello!
I am an experienced trainer and I have trained pokes from low lvl to high lvl.The training way or trick I used was:
Battle Tower Trick
There are three towers which are very good for training.

Things you need:
A team of Pokemons ( Ghost pokes like Spiritomb will be the best )
Items like Leftovers ( if you want to )
Patience ( the most important thing )

Go to Battle Towers.

You will see many towers.Forest Tower ( 4x of you'r highest lvl ) Marine Tower ( 5x you'r highest lvl ) Lava Tower ( 6x you'r highest lvl )

Forest Tower
It will gain less Exp but is easy to beat.
Use the moves"Blue Flare" and "Brave Bird"

Marine Tower
It will gain more Exp than Forest Tower but is harder to beat
Use the moves "Bolt Strike" and "Blue Flare"
Lava Tower
It is the hardest tower to beat you can gain tonnes of Exp from this one.
Use the Moves "Hydro Pump" and "Blizzard"

Currently the tower I use is the Marine Tower.

NPCs

I won't mention all the NPCs, not all of them are good for Exp.

Swimmer Richard.

Swimmer Richard is the most commonly used NPC. Use Bolt Strike against it.It has one pokemon and is 2x you'r highest lvl

Dragon Tamer.

This is also a good NPC. It is 3x you'r highest lvl use Blizzard against it. It has 2 pokes ( Not Sure)

Hope it helped...
Will add more soon!

Nice guide!

Ty I will add more soon

This message is hidden because this person has been banned.

Lol It's just an option you can use normal pokemons too! But Ghost pokes are better

Hmm Nice one =].

Ty :).........