You'll need lots of power items, pokerus, a Sweet Scent user, and a move that hits all pokemon on the field. With Pokerus, a regular Horde gives you 10 EV's in it's respective stat, and the power item gives you 40. So you can get 5 pokemon 50 EV's each per battle.
3DS FC: 1134-8061-2200; SV 3273; IGN: Dano; List: http://tinyurl.com/meg2s5l Beartic, Piloswine, and Snorunt - My Friend Safari is Ice because I'm so cool.
Step 1: Get something with Sweet Scent Step 2: Get something with Surf Step 3: Get Pokerus Step 4: Turn the EXP. Share on Step 5: Give the Pokemon you're training the appropriate Power item Step 6: Go to the route with the EVs you need and use Sweet Scent Step 7+: Use Surf
I was once modded for illegal activity because I made a topic asking for advice on nicknaming my Pokemon. 3DS FC: 5043-2277-6391 - THIEF
I didn't even try it until I had fully EV trained over 20 Pokemon. It is already faster and easier with just 1 Pokemon, but if you are training 4-5 then you save yourself a few hours.
Just get pokerus, give each Pokemon Power item item of the EV you want, go to an area that provides the right Pokemon in a horde (look up where in the guide), use sweet scent on one of your Pokemon, have your lead use Earthquake/Surf, and you get yourself 50 EVs in each battle. 10 battles, and 3 rounds in super training per pokemon and you're done.
The reason Horde Battles are so good is because using a power item adds 20 extra EVs for every battle, or 40 if you have pokeRUS. If you're training against a horde of 1 EV pokemon, that's 50 EVs per battle (with RUS). If it's a Horde of 2 EV mons, it's 60 EVs per battle. And you don't even need to use the pokemon you want to train because of EXP Share, just spam Surf/Earthquake/whatever with a high level mon.
In less than 5 minutes, you have a fully EV trained pokemon by just spamming Surf.