To get Cressilia, after defeating the Elite Four and getting the National Pokedex, go to Canalave City. Enter the sailor's house, and a boy will be in a bed. He will tell 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.