Ren has asked for help with battle strategies with his Pachirisu.

The Request

Check on the blackboard in Professor Laventon's office in Galaxy Hall in Jubilife Village to accept this request.

Talk to Ren

Go to the Crimson Mirelands and talk to Ren near the Mirelands Camp.

Do Battle with Only a Pachirisu

Remove all pokémon from your party except Pachirisu. Talk to Ren to battle against an Ursaring.

Reward

After you win, you get 3 Sitrus Berry and 1 Grit Gravel.