Read here how to take on Hala of the Elite Four!
Elite Four Hala uses the following Pokémon:
Thanks to its Thick Fat Ability, Haruyama receives half damage from Fire and Ice-type attacks.
Hariyama tends to use Fake Out on the first turn. Afer that, it often uses Knock Off if your Pokémon holds an item, and moves that are super-effective against your Pokémon.
Its Anger Point Ability increases Primeape's Attack to the maximum (six stages) if it is hit by a critical hit.
Primeape tends to use moves that are super-effective against your Pokémon. It's more likely to use Punishment if your Pokémon has lots of positive stat changes. Be careful of its Anger Point Ability, as it will deal lots of damage if triggered.
Its Fluffy Ability halves damage taken by contact moves, but doubles damage taken by Fire-type moves.
Bewear tends to use moves that are super-effective against your Pokémon.
Due to its Water Absorb Ability, being hit by a Water-type move will heal Poliwrath's HP, instead or receiving damage.
Poliwrath tends to use moves that are super-effective against your Pokémon.
Crabominable's Iron Fist Ability increases damage done by punch and certain other moves (in this case; Ice Hammer). With Fightinium Z, a single Close Combat can turn into the Z-Move All-Out Pummeling, which deals lots of damage.
Crabominable tends to use moves that are super-effective against your Pokémon. It may use its Z-Move if it cannot knock out your Pokémon otherwise.
Fairy is very useful as it hits all of Hala’s Pokémon for super-effective damage. It also resists Fighting- and Dark-type moves, meaning Hariyama (after the first turn), Primeape, and Bewear can’t deal lots of damage. In Primeape’s case, if it is locked into using Outrage, it will be useless against your Pokémon for a few turns.
Flying is also useful because it hits almost all Pokémon (except Crabominable) for super-effective damage. The only move it is weak to is Crabominable’s Ice Hammer. Fire is useful against Bewear and Crabominable.
Ghost can also help, as it is immune to Fighting-type moves and Fake out, but it needs to be careful of the Dark-type moves Hariyama, Primeape, and Bewear can use. Psychic can also be useful, but again needs to watch out for the Dark-type moves.
Hariyama has the same base Defense and Special Defense. Primeape and Poliwrath are slightly more weak to physical attacks. Bewear and Crabominable are more weak to special attacks, and most physical attacks make contact (and their damage is halved by Bewear’s Fluffy). Both would be the best option, but using just special is also good enough as the difference for Primeape/Poliwrath isn’t exremely important.
Granbull is a useful Fairy-type that can be caught late in the game in its evolved form. Mimikyu is another good Fairy-type that is also immune to Fighting-type attacks because of its Ghost typing. It is a bit rare to encounter, though. Alolan Ninetales is also a good Fairy-type that uses special attacks, but doesn't resist Fighting.
As for the starter Pokémon, Decidueye has the useful Ghost-type and access to the Flying-type move Brave Bird if it is above Level 55. It needs to be careful of Dark-type attacks, though, as well as Crabominable. It is particularly useful against Poliwrath. Incineroar’s Dark typing can be a problem, but with some care, it can take care of Bewear and Crabominable. Primarina is extremely useful for the entire battle, resisting all moves except Fake Out and Outrage.
Hariyama is the first Pokémon that will be sent out. Leading with a Ghost-type to avoid Fake Out isn’t too good of an idea because it may start with Knock Off instead. An interesting strategy would be leading with a Pokémon that isn’t weak to either Fighting or Dark, which increases the chances Hariyama will use Fake Out in the first turn, then switch out immediately to a Ghost-type. You will still have to be careful of Fake Out afterwards.
Alternatively, you may just let it use Fake Out successfully and just take it out. A good strategy would be using the move Acrobatics against it (the TM for it can be found in Ten Carat Hill) while your lead Pokémon doesn’t hold an item; this way you both deal double damage and don’t have to worry about double damage from Knock Off.
In any case, from the second turn onwards, you should just focus on taking Hariyama down as quickly as possible while still being careful with using Pokémon with held items against it.
Primeape is easy to handle since all of its moves cannot damage Fairy-type Pokémon a lot. If you have a Dragon-type Pokémon in your Pokémon League team, you may attempt sending it out in hopes that it will use Outrage. Immediately switch to your Fairy-type, and you got Primeape locked into using a move that your Pokémon is immune to for a few turns, which should be more than enough to take it out. You can even use the chance to setup on it, but at the same time you need to be careful when you have positive stat changes because it can use Punishment.
Bewear just needs to be taken down quickly. Hammer Arm reducing its Speed might be useful if your Pokémon couldn’t outspeed it the first turn (but may be able to on the second one, knocking it out). Keep the Fluffy Ability in mind.
Poliwrath also needs to be taken down quickly. Of course, you should not use Water-type moves against it (because of its Ability, but you shouldn’t be using them on a Pokémon that would resist them anyway).
Crabominable again needs to just be taken down quickly. If you are afraid of its Z-Move, switch to a Ghost-type Pokémon.
Also keep in mind that Hala, as well as all of the Elite Four, will use healing items at some point in the battle. He tends to use them if his Pokémon’s HP is low, so you may either want to try to take them down in a way that will make it unlikely for their HP to ever be low, or make sure it gets wasted early on so it isn’t used on a stronger Pokémon.
Overall, this shouldn’t be too hard of a battle thanks to type advantages and the very straightforward nature most of Hala’s Pokémon have.