After you beat the game go to solaceon town until you see professor elm. he will tell you about a dogish looking pokemon. then go to sandgem town and head inside the lab. you will see your rival and he will say that rowan went back to the spear pillar at mount coronet. go up there and talk to him. he will be with an old man. the old man will give you a clathentic orb. go to victory road and head out the secret exit(the guy will only let you go through if you get the national dex)go to the end of the route where that big stone is at. use the orb and it will break open. arceus will come out. then it will dissapear. go back to the snowpoint city and it will be there. press A again at it and it will leave again. go to wayard cave where mira waz at and battle it at level 70.