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.
Gulpin (Route 5 in grass) - 1 EV
Whismur (Connecting Cave) - 1 EV
Gulpin may have Liquid Ooze, so avoid Parabolic Charge.
Weepinbell and Arbok (Route 19 in grass) - 2 EVs each
Bellsprout and Ekans (Route 14 in grass) - 1 EV each
Durant (Terminus Cave) - 2 EVs
Nosepass (Route 10 in grass) - 1 EV
Nosepass may have Sturdy.
Psyduck and Roselia (Route 7 in grass) - 1 EV and 2 EVs
Spinda (Route 21 in grass) - 1 EV
Hoppip (Route 7 in grass) - 1 EV
Mime Jr. (Reflection Cave at exit) - 1 EV
Hoppip is immune to Earthquake.
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.