First u ether have to go to a pokemon event. Or u have to have a action replay with the Arceus cheat. After u get the flute u go to the place were u got Palkia or Dialga. The flute will make a noise it will ask u to play the flute pick yes. Stairs will appear. Go up them and there Arceus will be. He will be Lv. 80

He can learn every move except Focus Punch.