the ways i now are either when you see a bit of grass shaking there will be a pokemon if you battle it and win you will get a quite a lot of xp, you can also use rare candies and my last suggtion is the day care just offf stration city
hope this helped :)
You'll need to level it up to level 30.
According to my sources level 100
Have a friend with pokemon white 2 host the funfest mission
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
none of the fifth generation Pokemon learn it but in black and white sneasel learns it at level 42 houndoom learns it at level 26 and houndour learns it at level 25.
The only way I can think of is put them in the daycare.
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.
cheat
By giving it lots of rare candy.
Pokemon evolve when you level them up to a certain level, by trading them, by friendship, or by using stones.
you level it up 50
take a weak pokemon to the elite four
fight water Pokemon ( by fishing or surfing)
Get a female Combee and level it up to level 21.
You'll need to level it up to level 30.