There's already an active thread on this but... use the app on the Poketch to find Cresselia. It's the same thing with Mesprit, if you've caught Mesprit. I recommend switching routes between Route 209, I think it is, and the Valley Windworks. Then once Cresselia pops into the route you're in, you can use a Max Repel (Cresselia is unaffected by it) and track it down. To catch Cresselia, I used a Golbat's Mean Look + Quick Claw. But be careful, Golbat is weak to Cresselia's Aurora Beam. Once Golbat faints, Mean Look wears off and Cresselia will run in the blink of an eye. If that happens, just repeat the same strategy.
Hope that helped. ^^;