Without the right prep, Kayla will wipe you out.
But taunt will shut her down pretty fast. If you don’t set up taunt, you can get rid of her stat changes with clear smog. Her Sylveon and Scizor are annoying but they go down quickly with the right strategy. A good fire type like D. Chandelure takes care of Scizor easily, and any good Steel or Poison type shoots sylveon easily. Any dark type gets rid of espeon, and Gengar walls Mega Slowbro. For Blaziken, Water types beat it easily.
For Eduard, use taunt on his first Pokémon and then setup and sweep. Choice Scarf Delta Garde murders, so she is also a good choice. Delta Scyther also can sweep with Hustle and 1-2 hone claws. But be wary if you’re trying to set up because anything with half a decent attack stat and stab will on shot it. Aegislash stops both in their tracks however. So a good fire type would be a nice check, like delta Chandelure. Excadrill is also a problem for D. Garde, so Scyther may be a better choice.
Yuki can be difficult without the right prep. A good 1/2 of her team are weak to steel, but the rest are weak to ghost, save for Rotom-Freezer. Aegislash can counter most of her team though. But for Rotom-Freezer a good fire-type or Mold Breaker Excadrill would be a good choice. Hail is a problem because it chips a lots but potions outdo it’s damage. Not to mention Mega Cryoganal’s SLEET, which will kill you in 5 turns if you don’t have the right pokemon. Strong physical attackers get rid of it easily, not counting super effective damage.
Last is London. Your fast pokemon will have to take the backseat here. He uses trick room and can smash super fast teams. I recommend delta Zard or delta Milotic. Dark types work even better as their immune to most of his team. But be wary of his hard mode delta golurk. It’s fighting to get past dark types. Other than that this one isn’t too difficult. Freeze-Dry Users take out half the team, and a good fire type like D. Chandelure checks that scizor. Ghost/Dark types take out Mega Reuniculs and Jellicent. So Aegislash of Bisharp can prove useful in this battle.
The champion is Reukra and probably beats Cynthia in terms of difficulty.
But half of his team is killed by Choice Scarf Excadrill’s EQ, considering that you can get an HA Drilbur in Friend Safari right after the 8th gym. (1ofthe4rocketbros, in case you wanted to get it) Flygon will get walled by Mega Mawile, and Kyurem gets walled by any fairy, steel or fighting type. Shadow Mewtwo is murdered by bug types so bring a good one along for a ride. Considering that you can get a Spidergross before the elite four and a lot of Reukras team is also weak to ground, this is a pretty solid choice.
Let me know if anything in this guide should be changed. Thanks for reading!