This is a guide on how to take on Gladion on Mount Lanakila, before you take on the Pokémon League. You battle Gladion multiple times during the game, but we'll only discuss this one as it's the hardest one.
In his battle on Mount Lanakila, Gladion uses the following Pokémon:
The Inner Focus Ability prevents Crobat from flinching.
Crobat tends to use moves that are super-effective against your Pokémon.
The Pressure Ability makes Weavile's opponents lose 2 PP when using a move against it.
Weavile tends to use moves that are super-effective against your Pokémon. It also tends to use Ice Shard when your Pokémon’s HP is low.
With the Steelium Z Lucario is holding, a single Metal Claw turns into the Z-Move Corkscrew Crash, which deals lots of damage.
Lucario tends to use moves that are super-effective against your Pokémon. It also tends to use Quick Attack when your Pokémon’s HP is low, and may use its Z-Move if it can’t make your Pokémon faint otherwise.
Silvally’s type is determined by its RKS System Ability and the Memory item it is holding. The Memory item changes depending on which starter Pokémon you picked at the beginning of the game:
Silvally tends to use moves that are super-effective against your Pokémon.
Because Gladion does not specialize in a single Pokémon type, no single Pokémon type is viable against him. Instead you may use Pokémon of several types that can deal with some of his Pokémon.
Fighting is a useful type, being super-effective against both Weavile (4x weakness) and Lucario. Fire can also cover them both but is only 2x effective against Weavile.
Rock is a useful type if you picked Rowlet, as it is super-effective against Crobat and Fire Memory Silvally, as well as Weavile.
Electric is a useful type if you picked Litten, as it is super-effective against Crobat and Water Memory Silvally.
Ice is a useful type if you picked Popplio, as it is super-effective against Crobat and Grass Memory Silvally.
Crobat, Lucario and Silvally have as much Defense as Special Defense. Weavile is more susceptible to Physical attacks, but has a double weakness which means it doesn’t matter much if you use a Fighting-type move. In practice, both are good enough.
Kommo-o is a good Fighting-type to use.
Lycanroc is a good Rock-type to pair alongside Decidueye. It can be caught in Vast Poni Canyon so you don’t have to train one from a Rockruff; Decidueye itself may be able to handle Lucario, being immune to its Aura Sphere.
Any Pokémon of viable typing would be good in practice.
This battle is quite straightfoward; just try to knock everything out as quickly as possible using type advantages. Don’t worry too much about the rest of the strategy as these Pokémon only have attacking moves and are somewhat frail.
Overall, this should be an easy battle if you know which types to use at which point.