By battling other pokemon, use rare candy, or by putting the in the day care for a while.(note: if you leave a pokemon in the daycare for a while they may forget some moves you taught them)
cheat
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
To evolve Mantyke in pokemon black/white 2 you have to level it up with a remoraid in your party then it will evolve into mantine
Get a female Combee and level it up to level 21.
You can only level it up by, high friendship
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
the only way to get it is to level tynamo up to level 39
Level it up to level 50, then it will evolve into Zweilous. Level it up to level 64, to evolve it into Hydreigon.