This guide has been migrated here: http://www.floatzel.net/articles/horde.php

Once you get to the post-game, put your time into the Battle Maison instead of Super Training, then buy all the Power items. If you can, get yourself Pokérus (I got it through Wonder Trade). Also get yourself someone with with Sweet Scent and someone else with Earthquake or another all-attacking move (might need to be a fairly high level). Your Pokémon with the all-attacking move must be at the lead of your team. Sweet Scent may be on any Pokémon in your team.

Head to a good route (see the list below) and attempt to force hordes with Sweet Scent. Assuming you're in a battle with five Pokémon that yield 2 EVs each, something pretty wild will happen. Let's do a little math. 4 EVs from the Pokémon (2 base * 2 for Pokérus) + 8 EVs from the selected Power item (4 base * 2 for Pokérus) = 12 EVs per Pokémon. There are five wild Pokémon, so 12 * 5 = a whopping 60 EVs yielded from a single horde battle per Pokémon. If the horde only yields 1 EV per Pokémon, that's still 50 EVs per Pokémon. Halve these numbers if you lack Pokérus.

Let's also assume you want to train five Pokémon (with the last one having Sweet Scent plus an all-attacking move) to have maxed one specific stat but the rest of the stats to be maxed vary (doesn't really matter which as long as you have enough Power items to spread around). Due to the helpfulness of Exp. Share, everyone will get the EVs, with the distribution based on the Pokémon battled and the item equipped to each team member. Assuming you're training five Pokérus-infected Pokémon in a horde of Pokémon that each yield 2 EVs, that's 300 EVs gained in 48 seconds for each correct horde found.

As an added benefit, you have five chances of finding a shiny Pokémon.


BEST HORDE LOCATIONS

HP:
Gulpin (Route 5 in grass) - 1 EV
Whismur (Connecting Cave) - 1 EV
Gulpin may have Liquid Ooze, so avoid Parabolic Charge.

Attack:
Weepinbell and Arbok (Route 19 in grass) - 2 EVs each
Bellsprout and Ekans (Route 14 in grass) - 1 EV each

Defense:
Durant (Terminus Cave) - 2 EVs
Nosepass (Route 10 in grass) - 1 EV
Nosepass may have Sturdy.

Special Attack:
Psyduck and Roselia (Route 7 in grass) - 1 EV and 2 EVs
Spinda (Route 21 in grass) - 1 EV

Special Defense:
Hoppip (Route 7 in grass) - 1 EV
Mime Jr. (Reflection Cave at exit) - 1 EV
Hoppip is immune to Earthquake.

Speed:
Wingull and Taillow (Route 8 on water or Route 12 in grass) - 1 EV each
Floatzel (Victory Road at entrance) - 2 EVs
Wingull and Taillow are immune to Earthquake.

The first location listed is recommended due to frequency of appearing versus minimal extra EV gain. Sweet Scent does not work during foul weather such as rain, so use the second location if it is raining at the first location. In addition, EVs cap at 252 per stat (510 total), regardless of method used.

Now I just need to find someone with a real PKRS Pokémon in XY. xD

And spend a lot of time in the Battle Maison for all the BP I want.

Though...

I do find horde battles a drag.

I don't find them all that fantastic either. Especially since 1 turn takes 2 minutes for me.

If you still have really strong Pokemon from your story playthrough, then the Battle Maison is ridiculously easy. Or even better, go with the Speed Boost Blaziken, that's always overpowered. Select something you're very good at (Multi Battle for me) and stick it out for all 20 battles. 20 straight battles in the easy normal mode nets 48 BP, so doing that twice will net 96, just enough for all the Power Items.

Oh dang, I forgot Sweet Scent doesn't work during foul weather. I've added secondary locations for all of them in case of rain.

This guide is so much better than the one on Serebii, good(ra) job!

Thanks!

Looks like EVs cap at 252 per stat instead of the traditional 255.

Yaaay, a guide! I mean, I kinda did this already, but still nice to see it on paper! Just in case I forget and stuff. Yay!

I just realized, you said you were gonna do like a guide on Super Training. And you produce this. Nice follow up

I gave up on the Super Training guide because there's no reason to bother with it for massive EV gains (due to this). Of course, we'd still need some information on how to get stones.

Isn't there an evolution stone shop in Lumiose City?

If you're not maxing your stats out to 252 (If you're balancing between more than 2 stats) then Super Training is still useful. But this is speedy, so you got me there. I guess I'll add a little on for maxing out.

2 EV Battles w/ PKRS: 4 horde battles and 12 leftover EVs (1 regular battle.)
2 EV Battles w/o PKRS: 8 horde battles and 12 leftover EVs (2 regular battles.)
1 EV Battles w/ PKRS: 5 horde battles and 2 leftover EVs (1 regular battle without Power Item.)
1 EV Battles w/o PKRS: 10 horde battles and 2 leftover EVs (2 regular battles without Power Item.)

I don't think Dawn, Dusk, Sun and other stones can be bought.

Oh, I see what you're getting at now. There doesn't really need to be anything special on that, just complete the missions it tells you to really fast.

Vanillite and Smoochum from the Frost Cavern also yield Special Attack (1 each)
Also, no Taillow on Route 12

Taillow isn't on Route 12? It's been awhile since I've horde trained, so I'll need to check that out, then I shall fix it.

Wheres a great place to Ev Horde train in Sp.def?

I analyzed the OR/AS horde spots (link to pastebin) and found the following interesting ones:
HP: Whismur, Rusturf Tunnel (may have Soundproof)
Attack: Shuppet, Mt. Pyre (requires Surf to reach, immune to normal)
Defense: Sandshrew, Route 111 (may have Sand Veil, in case sandstorm; immune to Electric)
Special Attack: Oddish, Route 119
Special Defense: Swablu, Route 115 (immune to ground)
Speed: Zubat, Meteor Falls (immune to ground) or Zigzagoon+Taillow/Wingull, Route 104 (Taillow/Wingull immune to ground)
Note that this may be tentative
Edit: Can't Sweet Scent sandstorm