I think that is the only way to get it now. I don't think there was ever a shiny arceus event in the US. I have one, but only in Pokemon Black. I wish I could bring it over to ORAS, but Pokemon Bank doesn't allow hacked Pokemon to enter the Bank.
^^^ it does allow hacked pokemon if they look legit.i think the problem with old arceus is that from 4th gen from what ive read, the event item to catch it was never released or something, so arceus caught in that tower or hall place cant be obtained legit, youd have to get the item hacked.
Arceus can only be found in the wild legitimately with the Azure Flute, which was never distributed at any official event. However, Arceus itself was distributed via multiple Nintendo events in 2009 and 2010 and the Pokémon Global Link in 2012.
so if your arceus was in in the halls of origin then it kinda is legit but the azure flute used to open up that area must have been hacked..
have a look on bulbapedia for event distributions arceus.
my arceus from 4th is hacked and was caught in the halls of origin but i cant get it into poke bank from black 2