To start off, you will need the EXP share and a Pokemon to have Pokerus. Once you have Pokerus, spread it to the rest of the Pokemon you are E.V. training by battling several wild Pokemon until the desired Pokemon are infected (Please note that when spreading Pokerus to your team, you will need to have the EXP share off, or you could be giving the wrong E.V.'s to your entire team). Next, you need to purchase the appropriate power item, which can be found in the Battle Maison for 16 bp each, for every Pokemon you are going to train. If, for example, you are going to train 2 Pokemon in HP, you need to buy 2 Power Weights, and equip them to the Pokemon you are going to E.V. train. The Power Items give +4 E.V.'s to the stat they boost, and the Pokerus doubles the amount of E.V.'s gained (E.V.'s gained from battled Pokemon + 4 from power Item) *2 from Pokerus. Next, you need to catch a Pokemon that has Sweet Scent. It is a field move that when used in a patch of grass, you are guranteed to fight a horde of Pokemon. You can find an Oddish in Route 6 that has Sweet Scent. Finally, you could get a Pokemon that has an attack that can knockout multiple Pokemon at once (surf, rock slide, discharge, etc.). This isn't required, but it does make the battles go much quicker. Now, go to a route that has a horde battle that corresponds to the E.V.'s you want to train in, and go into the grass. Use the field move Sweet Scent, and you will be autmatically put into a horde battle. You may not always get the horde you want to fight, but the ones listed are the hordes you are most frequently going to find when using Sweet Scent. The following Routes contain hordes that give off specific E.V.'s:
HP: Route 5 - Gulpin
Attack: Route 14 - Bellsprout
Defense: Route 10 - Nosepass
Special Attack: Route 21 - Spinda
Special Defense: Route 7 - Hoppip
Speed: Route 8 - Wingull
How this works: Each of the listed Pokemon on its own gives off 1 E.V. on their corresponding stat once defeated. In a horde battle, you are going to find 5 of the same Pokemon ( exception of Minum/Plussle and Seviper/Zangoose hordes). So, if you battle a gulpin, you will get 1 E.V. towards HP. If you fight 5, you will get 5 E.V.'s in HP. If you fight them in the horde, you will get 5 E.V.'s off of the one battle. But, this is where the Power Items and Pokerus come into play. If you fight the 5 gulpin, while holding the Power Weight, you will gain 25 E.V.'s in HP, and if you have Pokerus, at the end of 1 battle, you will get 50 E.V.'s in HP. And the fun part is, if you have EXP Share on, and the rest of your Pokemon in your team have Pokerus and Power Weight equiped, then at the end of 1 horde battle with gulpin, all of the Pokemon in your team will each gain 50 E.V.'s in HP. After 5 horde battles, your Pokemon should each have 250 E.V.'s in the stat you are training in. To finish it off, meaning to get the stat to the desired 252 E.V.'s, simply go into the Level 1 Super Training session and beat it, instead of gaining +4, you should only gain + 2, meaning the stat has reached the 252 E.V. cap. Please note, that after 5 horde battles, you have to stop traing in that particular Stat, because unlike Super Training, you can roll over the 252 limit that Super Training imposes (the natural limit is 255), and you have to use a reset bag on all of your team if you mess up.
