There are two things you need. Sweet Scent to summon the hordes, and False Swipe to make sure you don't accidentally kill your shiny.
The choice of Pokémon isn't very large...actually, your only options are Mawile and Parasect.
Mawile can be found in the Glittering Cave, and...Paras/Parasect can't be found at all in X/Y.
If you don't wanna use Mawile, you could get two different Pokémon with Sweet Scent and False Swipe.
A good Pokémon I can recommend is Gallade. Gallade gets False Swipe, and some Status-inflicting moves. (It's easier to catch Pokémon if they have a Status!)
-Hypnosis (in case you find a shiny that can't be Paralyzed)
You'll need a type of Poké Ball to catch your shiny with...these are some good ones:
-Ultra Balls (probably the easiest one to get)
-Dusk Balls (if you're hunting at night or in a cave)
-Net Balls (if you're hunting for a Bug or Water type Pokémon)
-Timer Balls (in case the battle is really long)
Of course Master Balls are good too, but they're really hard to obtain unless you hack.
If you don't have access to a Sweet Scent Pokémon, you should stock up on Honey, because that's used to summon Hordes in the wild.
If you find a Shiny in a horde of Heatmor and Durant, or Zangoose and Seviper, you should probably know that...they'll try to kill eachother before attacking your Pokémon. To prevent the Shiny getting absolutely destroyed, you can....
-If you (somehow) have a Darkrai, use Dark Void on the horde and then take the non-shinies out one by one. If they wake up, just put them back to sleep again.
-Use Heal Pulse on the shiny whenever it's at low HP so it won't get KO'd.
-Skill Swap Levitate onto the Shiny and then proceed to Earthquake the rest of the Pokémon there. (This also works with Geodude hordes, since they'll often spam Magnitude on eachother)
Also! Because the camera angle on the horde changes often- make sure you have your sound on to make sure you can hear the sparkling noise when you find your Shiny...in case you accidentally kill it.
I hope this helped :D!