Read here how to take on Kahili of the Elite Four!
Elite Four Kahili uses the following Pokémon:
The Sturdy Ability prevents Skarmory from being knocked out in one attack (if it has full HP and an attack would make it faint, it will survive with 1 HP), and also makes it immune to one-hit knock-out moves.
Skarmory tends to use Steel Wing against Pokémon weak to it, and may also use Slash, Feint, or Spikes.
The Inner Focus Ability prevents Crobat from flinching.
Crobat tends to use moves that are super-effective against your Pokémon. It may also use Swift or Supersonic.
Its Dancer Ability makes Oricorio copy a dance move if your Pokémon uses it.
Oricorio tends to use moves super-effective against your Pokémon. It may also use Teeter Dance or Feather Dance, to confuse or weaken your Pokémon.
Its Big Pecks Ability prevents Mandibuzz' Defense from being lowered.
Mandibuzz tends to use moves that are super-effective against your Pokémon. It may prefer using Punishment if your Pokémon has positive stat changes. It can also use Flatter to not only confuse your Pokémon, but also give it two positive stat changes, making a combination with Punishment.
Thanks to its Skill Link Ability, Toucannon's moves that hit 2 to 5 times (in this case Bullet Seed and Rock Blast) will always hit 5 times. With its Flyinium Z, a single Beak Blast can turn into the Z-Move Supersonic Skystrike which deals lots of damage.
Toucannon tends to use moves that are super-effective against your Pokémon. It may also use Screech to reduce your Pokémon's Defense, making its attacks hit harder. Be careful of its Beak Blast causing a burn to your Pokémon.
Electric hits all of Kahili’s Pokémon for super-effective damage, and is only weak to Mandibuzz’ Bone Rush.
Rock hits all of Kahili’s Pokémon except Skarmory for super-effective damage, and is 4x effectiveness against Oricorio. However, it needs to be careful of Mandibuzz’ Bone Rush and Toucannon’s Bullet Seed.
Ice hits Crobat, Mandibuzz and Toucannon for super-effective damage, and is only weak to Toucannon’s Rock Blast.
Fire is the only type besides Electric that Skarmory is weak to. As Skarmory is hard to take down, it is quite important to use it if you don’t use Electric. It is neutral towards Crobat, and resists Oricorio’s Revelation Dance (but Oricorio resists Fire itself too). It needs to be careful of Mandibuzz’ Bone Rush and Toucannon’s Rock Blast.
Skarmory and Mandbiuzz are more susceptible to special attacks. Crobat, Oricorio, and Toucannon have equal base Defense and Special Defense. Special can make good use of Flatter and does not have to worry about Fearther Dance, the Attack reduction from a Beak Blast burn, and even the excess of confusion strategies in this battle (as confusion damage is proportional to your own Pokémon’s Attack). This means that special is strictly better in this battle.
Alolan Marowak is a good Fire-type overall and also very useful in other League battles. Its Ghost type makes it immune to the Normal-type attacks Skarmory and Crobat have. Salazzle is also a good Fire-type.
For Skarmory there are two options; either knock it out quickly to avoid getting lots of Spikes set up against you, or take advantage of the fact that it cannot deal lots of damage to set up yourself. Keep in mind that even with special super-effective moves (the best option against it), you cannot knock it out in one turn because of Sturdy.
Crobat’s strategy is quite straightfoward as well; you’d rather knock it out quickly (since it is frail), as Poison Fang can badly poison your Pokémon and Supersonic can confuse them. The combination of those and the occasional flinch from Air Slash (since Crobat is quite fast) can mean trouble. Steel-types resist or are immune to its attacks, so use that to your advantage if you have problems with it.
Oricorio should be able to be knocked out quickly as well; with special attacks Feather Dance isn’t a problem. Teeter Dance can be annoying but can be dealt with just like all the other confusion strategies. Avoid using dance moves if you don't want them used against you because of its Dancer Ability.
Mandibuzz’ strategy involves confusion as well, though Flatter has the added bonus of increasing your Special Attack by two stages which can be quite useful. It can be used against you with Punishment, so watch out, but you should be able to take Mandibuzz down quickly as well.
Toucannon is slow and you should take advantage of it by hitting it with a move before it starts using powerful attacks against you, such as its Bullet Seed/Rock Blast which will always hit 5 times. Its Beak Blast, while annoying, shouldn’t be too much of a problem if you use special attacks. You should be able to keep dealing damage to it without the burn damage over time bothering you much.
Keep in mind that Kahili, like all the Elite Four, uses healing items, particularly when her Pokémon’s HP is low. You may either try to make her Pokémon faint without ever giving her the chance to do so, or let her do it soon so that you won’t have to deal with it later, when she will be using more threatening Pokémon.
Overall, this shouldn’t be too hard of a battle thanks to type advantages, especially if you use items to get rid of the problems involving confusion.