Get the Azure Flute then go to spear pillar. it will ask you if you want to play the Azure flute. say yes then a staircase will appear. go up and you will see Arceus. NOTE: Save before battling.
if you know where it is buy lots of pokeballs because one doesnt always work. then when you find it lower its hp to red or yellow throw some pokeballs
An event that hasn't been held in America yet.It will probably be just getting the Pokemon instead of working for it, or a WiFi event.You get it at the top of Spear Pillar after playing the Azure Flute