The third grand trial is against Dark-type specialist Kahuna Nanu! Read here how to take him on.
Nanu uses the following Pokémon:
Sableye's Keen Eye Ability prevents its accuracy from being lowered.
Sableye tends to use Fake Out on the first turn. After that it tends to use moves that are super-effective against your Pokémon.
The Intimidate Ability reduces the opponent’s Attack by one stage when Krokorok is sent out.
Krokorok may use Swagger to confuse your Pokémon. If your Pokémon is faster and attacks first, but hurts itself from confusion instead, Assurance can be very powerful, so watch out. Crunch and Earthquake are also strong moves to watch out for. It tends to use moves that are super-effective against your Pokémon.
Its Fur Coat Ability halves damage done by physical moves, and with the Darkinium Z it's holding, a single Dark Pulse can be converted to the Z-Move Black Hole Eclipse, which deals a lot of damage.
Persian may use Fake Out at the first turn it is out. It tends to use moves that are super-effective against your Pokémon and may use its Z-Move if it seems like it can’t make your Pokémon faint otherwise.
Fairy is the most viable type as it is the only one that can hit Sableye for super-effective damage.
Bug can be useful as well, being super-effective to Dark and being able to use Buginium Z. Fighting is a bit less useful as it cannot damage Sableye, but is able to use Fightinium Z.
Water can also be a good option; using special Water attacks is similar to using physical Bug or Fighting attacks, as Krokorok is weak to it and Fur Coat means that a super-effective but physical move is in no way better than a special move that isn’t super-effective. At this point in the game, it is not very hard to use a powerful special Hydro Vortex.
Because of Persian being the hardest Pokémon to defeat, and it having the Fur Coat Ability, special is the way to go.
Ribombee has a very good typing for this matchup and access to special Bug moves, which can be combined well alongside Buginium Z.
Alolan Ninetales is another Fairy-type option with good offensive stats (though this Pokémon is only available in Pokémon Sun).
Any Pokémon of viable typing that can use special moves is useful for this battle.
As for the starter Pokémon, Decidueye is at a type disadvantage but can at least damage Krokorok super-effectively and prevent Fake Out from working, so it can then try to take the first two Pokémon down. Incineroar has to be careful of Krokorok but is otherwise fine. Primarina is very useful because it can damage all three Pokémon super-effectively and has access to Oceanic Operetta as well as Hydro Vortex.
You can use a Ghost-type Pokémon to avoid getting hit by Fake Out, and you can use a Pokémon that uses special attacks to make Krokorok’s Intimidate useless.
You can use Krokorok’s Swagger to your advantage, if you wish; it may confuse your Pokémon but it also increases its Attack by two stages, effectively doubling the damage its physical attacks deal. You can then proceed to use an item that cures confusion, knock Krokorok out and use a strong physical Z-Move on Persian. Fur Coat still halves the damage, but Swagger doubles the damage, negating each other out and allowing you to damage Persian normally.
Overall, if you use Fairy-type special attacks and the correct Z-Moves, this is an easy battle.