First:Raise your Pokemon to lvl 60+.
Second:Go to Gateon Port.
Third;Go to the top of the lighthouse,Mirror B should be there.
Fourth:talk to him,and he'll challenge you.
Fifth:Defeat his Ludicolo(which are quite annoying to fight).
Sixth:Try your luck at catching Shadow Dragonite...I would buy A LOT of Ultra Balls,it
took me a while before I caught it,but it was worth it.

DO NOT defeat it!!!
I dont know if it can be caught again!!!