Astral/H3X
Member
Hordes have 5 of the same Poke or 4 + 1 (so like maybe 4 Bulbasaurs, 1 Pikachu just to set an example)
Pokerus and proper Power Item equipped gets you 10, 12 ,14 EVs depending on how much EVs the Pokemon gives.
So multiply that shit with 5 and you get 50 Speed EVs from a horde of 5 Wingulls (1 Speed EVs), 60 Attack EVs from a horde of 5 Weepinbells (2 Attack EVs).
Lead with a Pokemon that knows an AoE move like Surf or Earthquake. Be in the grass and use Sweet Scent. Have Exp Share on so you don't have to bother switching in the Pokemon that needs to be EV trained.
That... Sounds amazing. Of course, you still have the issue of needing to make sure that you've got the right Pokemon to go up against, but still. Might need to get these Power Items...