After you have defeated the Elite Four, there is one more challenger for you to defeat: Professor Kukui!
Professor Kukui uses the following five Pokémon, as well as the final evolution of the starter Pokémon whose type is strong against yours:
If your starter Pokémon is Rowlet:
If your starter Pokémon is Litten:
If your starter Pokémon is Popplio:
Its Keen Eye Ability prevents Lycanroc's accuracy from being lowered.
Lycanroc tends to use moves that are super-effective against your Pokémon. It often uses Accelerock if your Pokémon’s HP is low. It may also use Stealth Rock once.
Its Snow Cloak Ability increases Ninetales' evasion in hail.
Ninetales tends to use moves that are super-effective against your Pokémon. It often uses Ice Shard if your Pokémon’s HP is low. It may also use Safeguard to protect itself from status conditions.
It has the Ability Keen Eye, just like Lycanroc.
Braviary tends to use Brave Bird on Pokémon that are weak against it. It may also use Crush Claw, Tailwind, or Whirlwind.
Its Sturdy Ability makes Magnezone survive attacks that would make it faint from full health with 1 HP, and also makes it immune to one-hit knock-out attacks.
Magnezone tends to use moves that are super-effective against your Pokémon. It may also use Thunder Wave or Mirror Coat.
Thanks to its Thick Fat, Snorlax receives half the damage from Fire and Ice-type attacks.
Snorlax tends to use moves that are super-effective against your Pokémon. It may also use Body Slam.
Incineroar's Blaze Ability increases damage done by its Fire-type moves when its HP is low. With its Firium Z, a single Flare Blitz turns into the Z-Move Inferno Overdrive which deals lots of damage.
Incineroar 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.
Primarina's Torrent Ability increases damage done by its Water-type moves when its HP is low. With its Waterium Z, a single Sparkling Aria or Aqua Jet turns into the Z-Move Hydro Vortex which deals lots of damage (less if used with Aqua Jet).
Primarina tends to use moves that are super-effective against your Pokémon. It may also use Hyper Voice, and may use its Z-Move if it cannot knock out your Pokémon otherwise.
Decidueye's Overgrow Ability increases damage done by its Grass-type moves when its HP is low. With its Grassium Z, a single Leaf Blade turns into the Z-Move Bloom Doom which deals lots of damage.
Decidueye 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.
Since Kukui uses lots of types, each Pokémon type is only viable against some of his Pokémon. In addition to that, because of one of his Pokémon being different based on your starter Pokémon, some viable types change accordingly.
Fighting is always good, being super-effective against Lycanroc, Magnezone and Snorlax. If your starter is Rowlet, it is also super-effective against Incineroar.
Fire is super-effective against Ninetales and Magnezone. If your starter is Popplio, it is also super-effective against Decidueye.
Ground is double super-effective against Magnezone and super-effective against Lycanroc. If your starter is Rowlet, it is also super-effective against Incineroar.
Rock is super-effective against Ninetales and Braviary. If your starter is Rowlet, it is also super-effective against Incineroar.
If you picked Rowlet, Water is useful against Lycanroc and Incineroar, and Fairy can be used to attempt to lock Incineroar into Outrage.
If you picked Litten, Poison is useful as it's super-effective against Ninetales and Primarina, and Electric is super-effective against Braviary and Primarina.
If you picked Popplio, Ice can be a good choice as it's super-effective against Braviary and Decidueye.
Lycanroc, Braviary, and Incineroar have equal base Defense and Special Defense. Ninetales, Snorlax, Primarina, and Decidueye are more susceptible to physical attacks. Magnezone is more susceptible to special attacks, but if you use Ground-type moves against it, it won’t matter much, not to mention that special attacks make you vulnerable to Mirror Coat. In any case, physical is better to use in this battle.
Kommo-o is a good Fighting-type to use, Alolan Dugtrio a good Ground-type (and its Steel type can be useful as well, especially against Ninetales), and Lycanroc a good Rock-type. These three are especially useful if you have to fight against Incineroar.
If you have to take on Primarina, Alolan Golem is extra useful with its Electric/Rock typing.
Alolan Marowak helps against Decidueye and can also use Bonemerang/Bone Rush to bypass Magnezone’s Sturdy and knock it out with 4x effective move in one turn.
If you use Decidueye, it is good against Lycanroc and Snorlax, but has to be careful of Crunch in both cases. The aforementioned Kommo-o, Alolan Dugtrio and Lycanroc are very viable as teammates. Ash-Greninja and Wishiwashi are good Water-types to go along with it, even if not physical-oriented. For the Outrage Lock strategy on Incineroar, Granbull would be a nice physical-oriented Fairy-type to use. Mimikyu can also work if you don’t mind having two Ghost-types on your team, or you can just lock it into Outrage and tank them with a Steel-type, bypassing the Fairy-type completely. In that case, Alolan Dugtrio is just as good.
Incineroar itself, being a Fire-type, is very good to use. Alolan Golem is a nice teammate option as stated above.
Primarina can deal with Lycanroc very well, as well as Ninetales, resisting its attacks. Crabominable is a good Fighting and Ice-type to partner with it. Alolan Sandslash is a good physical Ice-type that is also Steel-type, which can be useful in this battle, especially against Ninetales.
You may try to take Lycanroc down as quickly as possible in an attempt to prevent it from setting up Stealth Rock. Lycanroc’s strategy is very straightfoward as it is a fast but frail Pokémon, with mostly attacking moves. If it does set up Stealth Rock, be careful when you send in Pokémon weak to Rock. The moves Rapid Spin and Defog can get rid of Stealth Rock, but for the purposes of an in-game playthrough they are not very useful.
Ninetales’ strategy is straightforward as well. Of course you should not set hail for it while taking it down (preferably with physical attacks).
Braviary might be annoying if it keeps using Whirlwind on your Pokémon, especially since it is a bit hard to get viable Pokémon that outspeed it. As long as the Pokémon sent out isn’t particularly weak to any of its moves, you can consider wearing it down with attacks of regular effectiveness. It is a good idea to try and get Kukui to waste his healing items before having to face Braviary. Tailwind may also be set up, but there is not much you can do about it; just try to take advantage of the turn to deal some damage.
Magnezone is the classic annoying Sturdy Pokémon you have faced several times already. The Alolan Marowak Bonemerang/Bone Rush strategy can help, else just use a Ground-type Pokémon that is immune to Thunder Wave while you take it down in two turns. It can then only use the not-so-threatening Flash Cannon on you.
Snorlax should be taken down with Fighting-type attacks. None of its moves are super-effective against Fighting, although you should be careful as it hits hard enough anyway.
The last Pokémon depends on which starter you picked, of course.
Incineroar has Outrage, so if you can find a way to lock it into an Outrage and switch into a Fairy-type (that should have a super-effective move against it) you are set. You can also take advantage of the fact that its base Speed is very slow.
There aren’t any tricks to lock Primarina, so you just need to take it down in a straightforward fashion.
In Decidueye’s case, you could theoretically take advantage of a failing Sucker Punch, but sending in Pokémon weak to Dark (Ghost- or Psychic-type) wouldn’t work as well as in other cases of Sucker Punch-using Pokémon, as it may then use Spirit Shackle instead.
Keep in mind that Kukui, like the Elite Four, uses healing items, particularly when his Pokémon’s HP is low. Trying to make his Pokémon faint without ever giving him the chance to do so is practically impossible, so let him do it soon so that you won’t have to deal with it later, when he will be using more threatening Pokémon.
Overall, this should be a challenging battle, which is fitting as Kukui is your last obstacle before becoming a Champion. But even this battle can be made easier with correct use of type advantages and while keeping certain strategies in mind.