After defeating the Elite Four and getting the National Pokedex, go to Canalave City. Go inside the sailor's house and a boy will be in a bed. He tells you about Darkness and his mother will say he needs an item. Go outside to the dock. Talk to the sailor and catch a ride to Fullmoon Isle. Cressilia will flee and leave behind the item. Catch Cressilia the same way as Mesprit. Cressilia is at level 50.