Train it by using it in battle or if you have the Exp. Share, let it hold that and it will gain experience without having to be used in battle. Once it gains enough experience it will reach the next level.
Jiggly puff does not evolve at a level, but by using the moonstone.
it evolves to jiggly puff
Jigglypuff need a moon stone to evolve
In route 3 in the grass there are Jiggly Puffs
The first level you can atch Jigglypuff is route 3 in Chapter 2
In safari zone
Route112
jiggly puff
Route 115
sing
i believe you have to do 350 Brawls
350 matches in brawl--Nobody