With 510 EVs to gain for each Pokemon before they’re at their full potential, you’ve got your work cut out for you. Fortunately, there are ways to speed up EV gain.
Firstly, you can give your Pokemon certain hold items that promote EV gain. At the Battle Royal Dome, you can purchase Power items, such as the Power Anklet, for 16 BP apiece. Alternatively, you can get them for free by completing an easy sidequest outside the dome and from the postgame. These will add a flat +8 EVs of the stat they correspond to.
In addition, Pokemon that have contracted Pokerus will permanently earn double EVs, even after Pokerus is cured. You have a lottery-chance of contracting Pokerus from wild Pokemon, but thanks to the advent of Wonder Trade, it’s not uncommon for infected Pokemon to be sent en masse. So give Wonder Trade a try!
The best thing is that Power items and Pokerus stack. So if you fight a Pokemon that gives +1 EV to Speed with a Power Anklet equipped and the Pokemon has Pokerus, you will get (1 + 8) * 2 = 18 Speed EVs. Wow!
Additionally, if you’re EV training multiple Pokemon, you can turn on EXP Share, which shares out the same amount of EVs to your entire party. Whoa! Just make sure every Pokemon in the party is either already maxed out or needs the EV of that particular stat.
Finally, although Pokemon Ultra Sun and Ultra Moon do not have horde battles (the best method of EV training back then), you can use ally chaining for a similar effect. When a wild Pokemon is low on health, it may call for an ally to help it. You can use the Adrenaline Orb item to encourage this behaviour as well.
How ally chaining works is that after a Pokemon calls for an ally, you defeat the original Pokemon or the ally Pokemon–ideally the latter–and then wait for the remaining Pokemon to call for another ally. Afterwards, repeat the process, to spawn more and more allies.
Each ally Pokemon summoned grants double the usual EVs–and the best part is that this effect stacks with Pokerus and the Power items. So with each ally Pokemon defeated, you can gain (1 + 8) * 2 * 2 = 36 EVs. So to reach 252 EVs, you need only defeat 7 ally Pokemon. Not bad, not bad!
Recommended Pokémon to Chain
|HP||Caterpie||Route 1; grass near Iki Town.|
|Attack||Crabrawler||Randomly for any berry tree.|
|Defense||Pineco||Route 10; ambush from the trees.|
|Sp. Attack||Zorua/Magnemite||Grass in the Trainer's School.|
|Sp. Defense||Kecleon||Dividing Peaks Tunnel; from the shadows.|
|Speed||Diglett (Alolan)||Route 5 (north); from the small dens.|
Other Benefits of Chaining
Furthermore, as the "chain" of allies increases, the better the chances of finding Pokemon with high IVs, a rare Hidden Ability or even Shiny Pokemon!
To stop the chain from spiralling out of control, use a move that immobilises the wild Pokemon. In particular, Thunder Wave is perfect for non-Electric wild Pokemon, while moves like Hypnosis or Sleep Powder are effective against most Pokemon.