I got it to of using rain dance and it didn't evolve
You have to evolve it on a Route that is raining at level 50+. For example It rains quite a bit on route 14. Just Give it a rare candy or level up there once its raining.
At level 50 when it is raining. Source: Sliggoo pokedex in database http://pokemondb.net/pokedex/sliggoo
