Because Pokémon Legends: Arceus features a story centered around interacting with Pokémon in their natural habitats and building things from the ground up, most of the game’s missions revolve around catching or finding certain species.
In The Search for Bitter Leaves request, Shinon needs help making new types of medicine and asks you to help collect the necessary materials to do so. But in order to do that, you will need to track down a Petilil.
After talking to the head of the Medical Corps, Anise, for more information, you will need to head out to the Crimson Mirelands to bring back the “three-leafed little helper” in question.
You will need to trek all the way to the Holm of Trials and search the surrounding areas, where you should find a few Petilil hopping around near the water. You can also find Petilil in the Corronsedge Prairie. Once you manage to capture one, you can pack up and head back to Jubilife Village.
Speak to Anise again and give her the Petilil you just captured and she will start using its leaves to create the new medicine for Shinon, who is scared of Pokémon. For finishing the request, you will receive three Fine Remedy and a Hopo Berry.