How to beat the Grass-type Gym in Pokémon Scarlet and Violet?

While Pokémon Scarlet and Violet doesn't have a set order of which Gyms to challenge, following the order from the lowest to the highest level Gym leaders, Brassius's is the second recommended to do.

How to reach the Grass-type Gym?

The Grass-type Gym has no obstruction; just cross the South Province (Area 3), leaving Mesagoza by the East, and you'll reach Artazon; if you don't stray from the path, it is easy to get to.

Doing the Gym Test

The Artazon Gym Test requires you to find 10 Sunflora hidden all around town; some will try to fight you before following you; you can return them as soon as you find them or all at once; they are hidden all over the place, in alleys behind houses, in empty pots, even in areas where their color may blend with their surroundings.

You can play this minigame again at any point of the game, but you get a Leaf Stone when finding 10 and a Sun Stone when you find 20 of them.

Battling the Gym Leader

First Battle

Brassius specializes in Grass-type Pokémon, weak to Bug, Fire, Flying, Ice, and Poison-type moves and resistant to Electric, Grass, Ground, and Water-type moves.

His team comprises Petilil (Lv. 16), Smoliv (Lv. 16), and a Sudowoodo (Lv. 17) that he Terastalizes into Grass-type, Smoliv having Grass/Normal-Type adds a weakness to Fighting-type moves and immunity to Ghost-type moves. Sudowoodo will have the Sturdy ability, so even if you are overpowered, you'll require one extra turn to defeat it.

After the Battle, you'll receive a TM020 (Trailblaze).

Rematch

During the post-game, you have a rematch with him. His team is stronger this time, featuring Lilligant (Lv. 65), Breloom (Lv. 65), Tsareena (Lv. 65), Arboliva (Lv. 65), and Sudowoodo (Lv. 66).

Breloom has the ability Effect Spore, which may inflict poison, sleep, or paralysis on your Pokémon if you use physical moves; it is Grass/Fighting-type, removing the Bug-type weakness but increasing the Flying-type weakness and adding Fairy and Psychic to the list, while Dark and Rock-type are also added to the resistances.

Arboliva will summon a Grassy Terrain when hit, healing all Pokémon out every turn and boosting Grass-type moves.