Where can I train my Pokemon for the Elite Four?

Here are some good places:

• Battle Chateau. You may even run into some Gym Leaders after becoming Marquis(male) or Machioness(female) rank. Or trainers with a whole team of Audino, which gives a huge chunk of EXP points.

• Restaurants in Lumiose City. It is recommended that you choose either a Triple Battle or a Rotation Battle. If you choose Double Battle, you'll only fight two Pokemon per trainer instead of 3, and you'll miss out on fighting Pokemon like a lvl 63 Audino.

Beside that, there are a few techniques you can use to boost the incoming experience points.

• Give your Pokemon the Lucky Egg. The Lucky Egg is a held item that will increase the experience points given to the Pokemon at the end of battle. This item can be found in the Coumarine City Hotel and occasionally held by Chansey (Friend Safari).

• Increase your Pokemon's affection in Pokemon-Amie. Another way to get extra experience points from any Pokemon in your game is to increase the Pokemon's affection in Pokemon-Amie. Play games with it, pet it, and feed it until the hearts are filled up. You can check a Pokemon's Pokemon-Amie stats when you try to switch to another Pokemon at the Pokemon-Amie menu.

• Use the Exp. Point Power. If you use this O-Power, Pokemon will obtain even more experience points after battles, but only for three minutes after using the O-Power. This O-Power can be obtained in the Anistar City Pokemon Center.

Source IGN.