after you beat groudon just keep battling missions for a really long time then you'll talk to whiscash and he'll give you dive. Then you must beat a certain mission, but the Pokemon in the dungeon are very strong.
i don't know where in other games except Pokemon black and white sorry anyway after u beat the elite 4 go to unbella town and near a sine there should be a girl talk to her and she will give u dive...
This took me AGES to find out. Okay, so first, go to Retazo City, and head west until you come to that path where the lady is (the path you take to Mt. Pyre). If you haven't already done the Dark...