Arven asks you to investigate the Stony Cliff Titan in South Province (Area Three).
How to Find the Stony Cliff Titan
Go east from Mesagoza and you will reach South Province (Area Three). Keep going east along the path to reach the Pokémon Center of the area, and keep going east along the path until you see a watchtower. East of that watchtower, there is a crossroads, and if you look north from there, you will see a giant Pokémon on the wall. This is the Stony Cliff Titan.
Battle the Stony Cliff Titan
When you approach the giant Pokémon, it will go up and around the back of the rock, so go east and up the curving slope until you can see the giant Pokémon on the wall to the left. Approach it and you will battle Klawf, the Stony Cliff Titan. When its HP is low, it will run away. Go down to where it went, and there will be a cutscene, and then you and Arven will have a double battle against it. Arven has a level 16 Shellder.
After you defeat it, there will be a cutscene, and you will get the Sweet Herba Mystica. During the cutscene, you can choose to give your sandwich to Koraidon/Miraidon. Regardless of your choice, the Pokémon will gain the ability to dash. You can push the Left Stick like a button to dash, and you can dash into trees to knock Pokémon down.
When you are ready to move on, go south to the path through South Province (Area Three).