Read here how to take on Acerola of the Elite Four!
Elite Four Acerola uses the following Pokémon:
The Keen Eye Ability prevents Sableye's accuracy from being lowered.
Sableye tends to use Fake Out the first turn. After that, it mainly uses moves that are super-effective against your Pokémon. It may also use Confuse Ray to confuse your Pokémon.
Drifblim's Aftermath Ability damages the Pokémon that makes it faint.
Drifblim may use Ominous Wind to attack and potentially increase its stats, or set up Focus Energy/Amnesia. It then tends to pass all these stat boosts to one of the other Pokémon with Baton Pass. This can be a very deadly strategy.
Its Steelworker Ability increases the damage Dhelmise deals with Steel-type moves. As Dhelmise doesn't have any, this Ability is useless for this battle.
Dhelmise tends to use moves that are super-effective against your Pokémon. It may also use Slam, or Whirlpool to trap you.
Its Snow Cloak Ability increases Froslass' evasion in hail. However, this (alongside Blizzard’s hail effect) is useless unless you set up hail.
Froslass tends to use moves that are super-effective against your Pokémon. It may use Ice Shard if your Pokémon’s HP is low, and it may also use Confuse Ray to confuse your Pokémon.
Its Water Compaction Ability raises its Defense by two stages whenever it is hit by a Water-type move. With the Ghostium Z Palossand can use the Z-Move Never-Ending Nightmare.
Palossand tends to use moves that are super-effective against your Pokémon. It often uses Giga Drain when its HP is low, and it may also use Iron Defense to raise its Defense.
Dark is the only type (aside from Ghost itself) that hits Ghost for super-effective damage. It will not damage Sableye super-effectively, though. Acerola’s Pokémon have no super-effective moves against it.
Ghost can also damage all of her Pokémon super-effectively (again with the exception of Sableye) but it is also weak to their Ghost attacks in return.
Fairy is the only type that Sableye is weak to. It resists Ghost which makes it useful for the rest of the battle as well, and Acerola’s Pokémon have no super-effective moves against it.
Ice is also extremely useful as Drifblim, Dhelmise, and Palossand are all weak to it. It also resists Froslass’ Ice attacks, but Froslass resists those too in return. It is neutral towards Sableye, as are its moves against it. Acerola’s Pokémon have no super-effective moves against it either.
Fire is useful against Dhelmise and Froslass, and it only has to worry about Palossand’s Earth Power. It is neutral towards Sableye and Drifblim.
Rock is also useful against Drifblim and Froslass, but struggles against Dhelmise and Palossand. It is neutral towards Sableye.
Normal can be useful as well, being immune to Ghost. Normal-type Pokémon will have to use moves of other types (non-STAB), though, but Acerola’s Pokémon have no super-effective moves against it.
Sableye and Dhelmise are a bit more susceptible to special attacks, and Palossand is much more susceptible to them, with Iron Defense (and Water Compaction if you want to use Water-type moves on it) making the gap more significant. Drifblim is the opposite; more susceptible to physical attacks, and Amnesia can make this gap even more significant. Froslass has equal base Defense and Special Defense. Overall, special is better, but you need to take care of Drifblim as well.
Ash-Greninja is a very powerul Dark-type that also resists Froslass’ Ice attacks. It can use Night Slash and Dark Pulse to cover both physical and special Dark-type moves. Keep in mind that if you use an empowered Water Shuriken on Palossand, Water Compaction will take effect for each hit (although it will not matter at all if you intend on taking it down with just special moves).
Mimikyu combines two viable types (Ghost/Fairy) and is also useful in other parts of the game, although it is rare to encounter.
Alolan Ninetales is a good Fairy-type that uses special attacks and also has the viable Ice type. Granbull is another good Fairy-type that is useful a bit earlier in the game as well, and can be caught as its evolved form late enough in the game. It is better for physical attacks, though.
Alolan Marowak also combines two viable types (Fire/Ghost) and is extremely useful for most of the game and several other Pokémon League battles.
Lycanroc is a useful Rock-type that can also be caught in Vast Poni Canyon.
When it comes to the starter Pokémon, Decidueye has the useful Ghost-type and a powerful Z-Move of said type, but needs to be careful as it is also weak to Ghost itself. Its Grass-type can be useful against Palossand, but its physical attacks might not be the best option. It needs to be careful of Drifblim and Froslass, but Dhelmise is not a problem for it. Incineroar is very good for this battle, combining two viable types, and its Dark-type Z-Move is very useful. Primarina can make good use of its Fairy typing, and its Water type makes it resist Froslass and defeat Palossand. Water Compaction and Iron Defense will not matter as it uses special attacks.
In this battle you may want to just take Sableye’s Fake Out head on; using Ghost-types to get around it isn’t that good of an idea, you’d rather keep them for later when they can land super-effective STAB hits. Leading with a Fairy-type is the best option, so that you can try to take it down as quickly as possible, as Confuse Ray is the most annoying move it can use.
Drifblim is a Pokémon you should take quick care of, else it may set up lots of boosts and Baton Pass them (more likely to Palossand, as it is the most threatening Pokémon after some Baton Passed boosts). Keep in mind that upon making it faint, its Aftermath Ability will hit you in return. Don't use trapping moves in an attempt to prevent Baton Pass, all Ghost-type Pokémon are immune to such effects.
Dhelmise’s strategy is quite straightfoward, just try to knock it out quickly. It and Palossand have lots of moves that have a chance of reducing your Pokémon’s Special Defense in hopes that they will deal more damage later, but it isn’t too much of a problem. If you believe your Pokémon has gotten its Special Defense reduced too much (which is a matter of luck anyway), you can just switch out and stop worrying about it.
Froslass isn’t too much of a problem unless you make the mistake of setting hail for it. Ice Shard shouldn’t be doing lots of damage, and Confuse Ray can be annoying but can be dealt with.
Palossand should be taken down with special attacks; it shouldn’t be too hard.
Most viable types have no super-effective moves to worry about, and as there are lots of viable types, this a generally easy battle.
Keep in mind that Acerola, 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, as long as you don’t let Drifblim set up too much and Baton Pass.