Instructions
Defeat the Elite Four and the Pokemon League Champion. Spot every Pokemon in the Sinnoh Pokedex and talk to Professor Oak to obtain the National Dex.
Fly to Canalave City and take the boat to reach Fullmoon Island. Enter the small clearing and talk to Cresselia, and it will flee.
Move a Pokemon between levels 40 and 50 to the front of your party and equip the Poketch Map. The skull-like icon on the map represents Cresselia's location.
Ride the Bicycle back and forth between adjacent routes such as Routes 212 and 213 until Cresselia's icon coincides with your location. Use a Max Repel and enter the grass to find Cresselia.