Best way to level up to level 100 fast?
(No day-care method)
3-star Restaurant Cafe Le Wow in Lumiose City. It costs 100,000 to participate, but you win more back with the prizes.
Give the Pokemon a Lucky Egg, and then use the Experience Gain O-Power. Take on the Triple or Rotation challenge. The 5 trainers use 3 Pokemon each, all level 65. I have taken on this challenge so many times I could probably tell you their Pokemon from memory. They give you a ton of experience (especially since the second battle has a level 63 Audino). If you only do Double Battles, then each trainer does not have the third Pokemon (meaning you miss out on Audino and Togekiss, which give tons of exp).
1. Ferrothorn, Druddigon, Qwilfish
2. Amoonguss, Alomomola, Audino
3. Milotic, Wobbuffet, Shieldon
4. Lucario, Floatzel, Togekiss
5. Aurorus, Exploud, Chatot
This method allowed me to train up a level 30 Dragonair and Pupitar to evolution to complete my Pokedex within 30 minutes. I don't know how effective it is for the rest of the levels up to 100, though.