You have to beat the league first.After you have seen all the Pokémon in the Unova Dex, you will receive a License from professer juniper. This item, when shown to the airport attendant in Mistralton City, takes you to the Nature Sanctuary, a place filled with wild Pokémon. However, in the middle of this sanctuary is the rare Shiny Haxorus.
Following these steps should help yu get shiny haxorus.!