This quest is at the ending of the main story. Post-ending. Ride Entei on Mitonga Road to smash boulders and up a slope, turn left you will see a tree. Make sure you have a pokemon with tackle field move like 1 or 3. Have that pokemon tackle the tree. A red berry will fall. Fly on Latias/Latios back to Cocona Village, go to Brooker's shop and give the red berry to Brooker. Quest Cleared. Watch a scene between Pros. Hastings and Brooker.