The last grand trial in the game is against Ground-type specialist Kahuna Hapu! Learn here how to take her on.
Hapu uses the following Pokémon:
The Sand Veil Ability increases Dugtrio’s evasion in a sandstorm.
Dugtrio may use Sandstorm to increase its evasion thanks to its Ability and damage all Pokémon that aren’t part Rock, Ground or Steel-type over time. It may also use one of its attacking moves, preferring to use one that is super-effective.
The Sticky Hold Ability prevents Gastrodon’s item from being taken away. It is practically irrelevant in this battle as Gastrodon isn't holding an item.
Gastrodon may use a move that’s super-effective; its attacks all reduce accuracy. It may also use Recover to heal itself when its HP is low.
The Levitate Ability makes it immune to Ground-type moves.
Flygon may prefer to use a super-effective attack.
Its Stamina Ability increases Mudsdale's Defense by one stage when hit, and with its Groundium Z, one Earthquake can turn into the Z-Move Tectonic Rage which deals a lot of damage.
Mudsdale prefers using super-effective attacks. Its Ability makes it receive less damage from physical attacks, which makes it easier for it to use Counter.
Grass is the only type super-effective against Gastrodon, and is also double super-effective. It also can use Grassium Z.
Ice is double super-effective against Flygon.
Water is super-effective against Dugtrio, unlike lots of other types super-effective to Ground that are not-very-effective against Steel. It also can use Waterium Z.
Gastrodon and Flygon are both double super-effective weak against a type so it does not matter. Dugtrio, and especially Mudsdale, are much more susceptible to special attacks.
Any Pokémon of a viable type is good for this battle.
When it comes to the starter Pokémon, Decidueye is very good just because of its type. Incineroar isn’t that good so it may need help from other teammates. Primarina is very good and has access to Oceanic Operetta.
Try to take down Dugtrio quickly, hopefully before it sets up a Sandstorm, triggering its Ability. It also might hit hard.
You’d rather just use a Grass-type move against Gastrodon, else it will take a few turns to bring it down.
Similarly, an Ice-type move against Flygon is the best option.
For Mudsdale, just use special attacks so that Stamina doesn’t matter and you don’t get hit by Counter. Once again, taking it down quickly to minimize the damage it will deal to you is key.
Overall, this is a pretty easy battle if you use type advantages correctly.