keep battling the elite four. lol jk get a life
the fastest way to level them up is to take them to an area that has Pokemon leveled at or slightly below the level that your Pokemon is also if you battke with exp. share on your Pokemon it gains double exp.
Easy. Give him or her an Experience Share, then battle a high level pokemon.
challenge the elite four or give Pokemon exp share _hope this helped
Level them up
The fastest possible way is to train a Pokemon someone else gave to you and have it hold lucky egg.
work with them a lot until they level up but don't let him or her faint very much or else they wont like you
use rare candy or rematch Giovanni over and over and over again.
the easiest way to level up your Pokemon to put it in daycare Pokemon will level up soon.Trade the Pokemon to friend and have it hold a lucky egg it will level up quickly.
Giving a Pokemon an experience share at a low level while other, stronger Pokemon fight will level up to a strong enough to level to fight well and train. You can also level them up faster by using Rare candies either by using cheat codes and/or action replays or saving up Rare candies found throughout the game. The best way to level up your Pokemon is to fight constantly although it is not the fastest way.
Well you can go back to an earlier route where weaker Pokemon are you can battle them and after a few battles you will level up not exactly the fastest way but it prevents your Pokemon from getting hurt, once your stronger defeat stronger wild Pokemon.
The easiest way to level up is to feed Pokemon Rare candies.
straight up SD