just keep practising with the Pokemon league until your Pokemon goes to level 100. Or If you haven't got to the league yet you can keep battling trainers and wild Pokemon.
you level can level up your Pokemon by given it an luckey egg when betteling
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.
To get a Pokemon level up quickly is to have a Pokemon hold exp. Share and then just battle a bunch of trainers or wild Pokemon.
By giving it lots of rare candy.
fight water Pokemon ( by fishing or surfing)
To up your Pokemon's to 100lv you have to fight the elite 4 100 time ......
Training a Pokemon that was traded to you gives it boosted experience points when you win also if a Pokemon holds the item lucky egg you receive more exp points.
Deposit more Pokemon
train, train and more training. or use the pokemon clone glitch and let your pokemon hold a rare candy for level up.
Have a Pokemon that was traded to you then have the Pokemon hold a lucky egg and start battling your Pokemon will level up very quickly.
Walking, running, surfing and biking all grant 1 experience point to each Pokemon in the day care for each tile you travel over. Because biking travels over more tiles more quickly than any other form of transportation, it will level Pokemon in the day care more quickly than the other forms of transportation.
Pick one of your pokemon to level up. Now don't spend money on anything else until you have lots of money. And don't level up your pokemon as soon as you can . Be pationte