What u have to do is battle a lot with your shadow pokemon and use the massage therapy stuff that u get from a special shop for pokemon

FIND A TIME FLUTE AND USE IT AT THE RELIC STONE AND......POP POKEMONS PURIFYD BUT ONLY ONE PER FLUTE.

thereare three ways of puryfying pokemon
no.1 you can battle with them a lot.
no.2 you can use massage oils NOTE(more effective when used in thatpokemons type zone in that place when you go in th lift downstairs.
no.3 you canuse a time flute instant purifacation.
hope this help pease ask more questions
longland28 over and out.