Arceus i the god of all pokemon. It can only be obtained with the Azure flute witch you can only get with nintendo events. After obtaining it, head to the Spear Pillar where you fought Dialga or Palkia. Go on the symbol btween the two pillars and some stairs will appear taking you up to the "Hall of Origin". Arceus is at lv 80 so b reparred. It knows

Hyper beam
Recover
Refresh
Future sight