Ren is looking for new ways to battle with his Pachirisu. That means he wants you to show him how to battle with his companion Pokémon by finding one of your own. Get ready for an intense battle. Here is how you complete the Battling with Pachirisu request in Pokémon Legends: Arceus.
You will unlock this request after you calm Ursaluna in the Crimson Mirelands. Return to Jubilife Village and check the blackboard to get the request. Afterward, you will need to meet up with Ren at the Mirelands Camp in the Crimson Mirelands.
Ren will task you with finding your own Pachirisu and doing battle with it against an Ursaring. It’s a pretty daunting task, but each step is easy to understand. Head southwest to the western side of Gapejaw Bog. Here you will find a lot of Pachirisu along with an alpha. If possible, catch the alpha as it will help you the most during the battle. Make sure to catch a few Pachirisu here since you can use multiple during the battle.
Return to Ren at the Mirelands Camp. Make sure to stop and remove all the Pokémon from your party besides the Pachirisu you caught. Talk to Ren and the battle will start. Once the battle is over, you will be rewarded. If you lose the battle, you can always try again.
