Instructions
Battle all the trainers in the various cities and capture and purify as many Shadow Pokemon as possible.
Check your "Miror Radar" to find the location of Miror B. Go the location and battle Miror B. He will have one or two Shadow Pokemon with him that you didn't catch while battling the trainers. Keep locating and battling Miror B until you have caught and purified 82 Shadow Pokemon.
Check your Miror Radar one more time. It will tell you that Miror B is at the top of Gateon Lighthouse.
Go to the Gateon Lighthouse for the last battle. He will have a level-50 Dragonite with him. Capture Dragonite during the battle.