Hold a Power Bracer for +8 Attack EVs per defeat. Power items stack with Pokerus, so each defeat gives the base EVs plus the 8 bonus, and the whole total is then doubled. Macho Brace doubles everything as well and stacks with Pokerus, it just costs you speed in battle.
EV training in Cobblemon
Effort values are hidden stats you gain by defeating Pokémon. Each species yields EVs in specific stats, so beating a Geodude gives Defense EVs. One Pokémon can hold 510 total EVs, with a maximum of 252 in any single stat.
Efficient EV training
Efficient training comes down to two things: a high EV yield and a species that is easy to find. The table pairs the yield data with spawn rarity, levels and biomes so you can pick a grinding spot instead of guessing. Power items add 8 EVs of their stat per defeat on top of the base yield.