Poke I have the X/Y guidebook, and according to it the best ways to level up pokemon are as followed:
Switch out the pokemon as soon as the battle begins
Revive the pokemon(if it fainted) before the battle ends
Trade it with someone to train it for you
Use the Exp. Point O-Power
Hold off evolving it
Play with your pokemon often in Pokemon-Amie.
That is all the guidebook says about leveling up weak pokemon. Good luck!