This request is initially picked up from the Request Board in Galaxy Hall. You’ll unlock it once you’ve got access to Ursaluna from Crimson Mirelands. When you pick up this quest you’ll be tasked with speaking to Ren, who’s found at Mirelands Camp in Crimson Mirelands. He’ll ask you to find and catch a Pachirisu (if you don’t already have one) and then battle his with only Pachirisu in your party!
Where to Find Pachirisu in Arceus
There are a couple of areas where you can find a Pachirisu in Crimson Mirelands. For example there’s a few southwest of Gapejaw Bog (there’s even an Alpha Pachirisu here!) and a few spawn near Cottensedge Prairie.
How to Battle with Only Pachirisu
Your first plan of action should be to use thunder wave - it doesn’t do any damage but it does paralyze your enemy and potentially stop them from taking their turn. Then you can whittle down the Ursaring using Play Rough and Crunch. If the Pachirisu you’ve caught it a bit lower levelled than the level 30 Ursaring then you might want to either stock up on potions or go out and do a bit of level grinding before hand.
Once you’ve defeated the Ursaring, speak to Ren to finish the request and earn x3 Sitrus Berry and x1 Grit Gravel. Don’t forget to return your Pokémon to your party before you head out again!