I was planning on catching it, trying to get it shiny. Also what are some good techniques to get shinies?
After receiving the Lunar Wing from the Stranger's House, head to Marvelous Bridge. You will be told to hold the Lunar Wing up high, and once you do, Cresselia will appear for battle.
Some strategies for Shinies:
Go to weird house get the wing go to marvelous bridge in the center. you can catch cresselia there. On tips with shinies the masuda method chaining and running around aimlessly in the wild. Shiny cresselia you really want! so save before you battle her and keep soft ressetting until it is shiny!
Cresselia is at Marvelous Bridge. Best way to get a shiny, soft restart then exit and back.