I'm assuming you're asking about the one in the Nature Preserve. First you need to beat the Elite Four and Champion. Next you need to see all 297 Pokémon in the Unova Pokédex besides the four event legendaries, Victini, Keldeo, Meloetta and Genesect. Once you've seen all of the Pokémon, go to professor Juniper's lab and she will give you the ticket that allows you to go to the Nature Preserve.
Then go to Mistralton City, enter the terminal building and speak to the person behind the counter. When you show them the ticket, you'll automatically be flown there. When you land there, you will start on the left edge of the Preserve. Walk down to the bottom of the screen and past the plane. Then walk to the right and up through the opening in the trees. This is where Haxorus is found.
Make sure you get a Pokémon you can lower its health (I recommend something with False Swipe) with and a lot of Poke balls. Save the game before you start the battle in case you accidentally KO Haxorus.
If you do KO it or wanna find a different shiny Haxorus, you'd have to breed or encounter Axews until you find your shiny, which will take a while because the chance of encountering a shiny in that Gen is 1/8,192.
Hope I helped!
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