It is a version exclusive for White, so you can not catch a wild one. In the Second Gym city you can trade a Cottonee for one, since it is trade you won't be able to name it, but it will get boosted exp.
Go to Nacrene City and there will be a house with stairs to the right of the Pokemon Center. Go into the house and there will be a girl upstairs and she will want a Cottonee for a Petilil.
Level 64
To evolve petilil you have to use a sun stone.
level 17
Level 37
Level 31.
Sun stone.Of course its the same with COTTONEE,someone will be willing to give you a cottonee/petilil in that town...With leonora
Petilil evolves with a sun stone.
Petilil can evolve into Lilligant at any level because it requires a Sun Stone to evolve.
Level 64
Level 43
Petilil evolves with a Sun stone, not by level. You can get one in Nimbasa City, as well as in the Relic Castle. Hope it helps
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
Audino doesnt evolve in Pokemon black and white
Throh don't evolve in Pokemon Black and White.
Virizion does not evolve.
Level it to 41 and it will evolve
watchog does not evolve