To get this legendary pokemon go to canalave city after you beat the game and head into the sailors house and you should see his kid is in his bed. talk to the kid and the mom then the sailor will come in and ask you to go to new moon island to retrieve medicine. go from his boat and go up to cresselia at new moon island. it will dissapear but grab the item then head back to canalave. the boy will be cured and then to get cresselia look on the poketech app. that you used to catch mesprit and start chasing down cresselia.