In Black or White 2 you go to the Strange House near Lentimas town and reversal mountain. after inside the house wander on the 3 floors, once you get to the top on the right you'll see a girl talking then she will disappear. You won't be able to go into the middle room to get the Lunar wing. Go around the house a little, soon you will be able to go into that room before you do you will see the girl again, then you can go into the middle room and find the Lunar wing, after you collect the wing take it and go to Marvelous Bridge. Run around a bit, soon you'll get stopped and it will ask if you want to raise it into the air, and then Cresselia will go in front of you, but bring a lot of poke-balls. It took me 15 to 25 to catch her. Hope I can help again.