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)
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.
To up your Pokemon's to 100lv you have to fight the elite 4 100 time ......
use 25 of golden seeds
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.
Silcoon will evolve at a low level because bug Pokemon evolve quickly.
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
The only way I can think of is put them in the daycare.