Pesselle has heard many stories about how Croagunk's venom can be used as medicine. Time to see if this story is true and heal an older man's back pain. Time to go Croagunk hunting. Here's how you complete Croagunk's Healing Poison request in Pokémon Legends: Arceus.
Before you can get this request, you will first need to complete the task of calming Ursaluna. Once done, you will be able to get this mission from Pesselle in the Galaxy Team HQ. He enters the infirmary which is to the right of Cillene's office. He talks to Pesselle and will ask you to bring her a Croagunk so that she can make a remedy from her poison.
You may have already found a Croagunk since advancing into the Crimson Myrene Lands. If not, head to the Crimson Mirelands and head to Gapejaw Bog. This is south of Mirelands Camp. Here you will find plenty of Croagunks to catch. Be careful, Croagunk has a wide range in which they can spot you and will attack you if they spot you.
Once you have a Croagunk, go back to Jubilife village and talk to Pesselle. He will ask about the Pokémon and make some medicine with her poison. She will reward you with three full heals and three little Exp candy.