Mallow is living in the restaurant along the west side of Konikoni City, just below Olivia’s shop. On the ground floor, there’s a punk girl by the staircase who has a present for you: a Whipped Dream and a Satchet. These items evolve Swirlix and Spritzee if they’re traded while holding the correct item.
Upstairs, you can bag yourself another present by chatting to the cook on the sofa: a Dragon Fang. This item boosts the power of Dragon-type moves when held, although the Draco Plate does the same thing.
If it’s currently day-time in the game, you can also meet Mallow inside her room. She’ll tell you to meet up at the Lush Jungle, where you undertook her trial, before running off ahead. Exit the restaurant and navigate Charizard to the Lush Jungle’s glide spot.
Inside the jungle, go over and interact with the lonely Wimpod near the entrance. Mallow will arrive and challenge you to a battle as you desired.
Ultra Sun players already battled Mallow during Mina’s trial; her team here is basically the same, but slightly stronger. The slow but powerful Trevenant can use Horn Leech and Shadow Claw for STAB, and Leech Seed to slowly steal HP. Fire, Flying, Ice and Dark-types are all great choices.
Shiinotic, which can be her second Pokémon, has good special-based stats, but pitiful Speed. For attacks, it can use Moonblast and Giga Drain for STAB, Spore to cause sleep with a 100% chance, and Ingrain to slowly restore HP. Poison-types are its bane; Fire, Flying, Ice and Steel-types should also do well.
Finally, Tsareena is a fully evolved Bounsweet and Steenee with excellent Attack, but average Speed. It can use Power Whip for STAB, Hi Jump Kick to pummel Steel-types, Zen Headbutt to punish Poison-types, and U-turn to switch out. It’s also holding Grassium Z. Fire or Flying-types are recommended.
Win against Mallow and you’ll get the inspiration for the Reverent Battle Style. Huh, it’s strange to think Kiawe and Mallow share something in common.