This mission, like many missions, is a battle. The objective for this one is simply to defeat all of the enemies. Marche and Montblanc automatically participate in this battle, but you are free to choose four more members of your clan to join in the battle. When the battle begins, you might want to start out by focusing your attacks on the Red Cap, because it has some pretty dangerous attacks, particularly Goblin Punch, which does random damage to its target. The Sprite is probably OK to leave for last, because it's really weak.
Once you're done with Herb Picking, you'll get some Gil for a reward, as well as Ability Points (which make all of the members of your clan who participated in the battle progress in whatever skill training they're doing). You'll also get some items.