like Cipher said you need to have snagged all previous 82 shadow pokemon, then your miror B radar will go off, head to the top of gateon port lighthouse, you will see Miror B dancing with his ludicolo to celebrate 'team Miror', walk up to Miror B and you fight him, all his team are Ludicolo except for Dragonite, after you defeat him he will yell at you for snagging the final shadow and he will storm off into the lift, he will then notice the miror B radar and break it.
then do what I did, dumped it in the purify chamber