By giving it lots of rare candy.
you level can level up your Pokemon by given it an luckey egg when betteling
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.
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.
fight water Pokemon ( by fishing or surfing)
To up your Pokemon's to 100lv you have to fight the elite 4 100 time ......
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.
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
there is a daycare, but it's not fast at all; every daycare gives 1 exp for every step you take. that's why you keep biking around to get eggs and to level up daycare pokemon. If you want to level up quickly, look up EV training and do that. When you EV train, leveling up will come with it
The only way I can think of is put them in the daycare.
The quickest way to level a pokemon up is to trade it to another game, and then begin to level it up. Traded pokemon have a multiplier that calculates with the experience points gained in a battle. You can also obtain the item, lucky egg.
The best legit way to quickly level up your Pokemon is by using a lucky egg, which doubles a Pokemon's exp after a battle. Or just use rare candies derived from an action replay if you are okay with cheating.