Grinding

For the first gym you have the option to battle against a training partner near the berry store. 

He can be rebattled as many times as you want to, till you have beaten the gym.

If you trained with him and talk to him after you beaten the gym, you'll get an Rare Candy.

For the second gym you have the option to battle against a training partner near the Pokémon Mart. 

She can be rebattled as many times as you want to, till you have beaten the gym.

If you trained with him and talk to him after you beaten the gym, you'll get vitamin item depending on your Trainer Class.

When you have beaten the second gym, you get access to the first Double Battle Institute Cup in Amberdale.

Here you can train your Pokémon in double battles as many times as you want to.

The cup is a little easier compared to the Sableye cup in the Storage Halls Colosseum.

This cup gives as all colosseum battles 3x exp.

In the process of the D&D sidequest "Hopeless geniuses" you will get access to the Storage Halls Colosseum, where you can challenge the Sableye Cup right away.

To start the quest, talk to D&D in the top left house in Granitedeep City.

For the forth gym, the Sableye Cup is still your best shot. It is recommended to clear the whole island before challenging the gyms in Topaz Oasis or Jade City, this will provide a lot of exp as well.

This cup gives as all colosseum battles 3x exp.

After you have beaten the forth gym (in the order you have choosen), you'll get access to the Solrock & Lunatone Cup in the Double Battle Institute.

Since you can fly at this point of the game, this is a valid option.

Anyway, the level gap between gym four and five is quite small, so you should be able to take the fifth gym without major problems.

This cup gives as all colosseum battles 3x exp.

After you have beaten the sixth gym, you'll get access to the Houndoom Cup in the Storage Halls Colosseum.

This cup gives as all colosseum battles 3x exp.

After you have beaten the seventh gym, you'll get access to the Espeon & Umbreon Cup in the Double Battle Institute.

This cup gives as all colosseum battles 3x exp.

After you have beaten the eight gym, you'll get access to the Drapion Cup in the Storage Halls Colosseum.

This cup gives as all colosseum battles 3x exp.