CRESSALIA is in all 3 games, how to get her is by finishing the game next is to get the National Pokedex then go to a place where there is at least 3 boats, 1 close to a House that is Locked(Aka: a 50 year old the Hotel) and 2 close to a Sailor's home.
Go inside the house and you will find the Sailor's son that has been hit by DARKARI's Nightmare, go to the Sailor and he will tell you what happened but not 100%,after the Story ended he will take you to a place called FULLMOON ISLAND.
Finally go in more inside the Island and you will see CRESSALIA go in front of her, press A and she will fly away leaving the Lunar Wing to Break the Nightmare from the Sailor's son and now have a Happy Hunting around Sinnoh. (P.S. you need Poketch app #13)