Go to Trademond's guardhouse and get to the top floor. Speak to Julian, the guard captain to get the quest.
Your objective is to defeat either the Bandit Leader or the Ogre Commander and report you got the bounty to Julian.
The bandit leader is located in the southern part of the Forest of Runes. You need to destroy a plant with Fire or collar removal to access this zone. To reach the boss, you have two options:
take the front door to the shack west of the bandit encampment,
destroy a rock east inside the southern Forest of Runes. You will be able to get inside a tunnel leading to a back attack on the boss.
The bandit leader has 500 HP, 19 DEF, 9 MDEF and the following combos: Prapare Bomb + Bomb and Windup + Pierce I. He is also accompanied by two forest bandits and one corrupt guard.
If you want to beat the bandit leader without removing Aura's collar, having a good setup is key. Clearing out his underlings quickly is also important. Waterskin is the best counter to bomb attacks. Here are some ways to complete a good setup.
The ogre commander is located in the northern part of the Forest of Runes. You need to destroy a rock with a bomb or collar removal to access this zone.
The ogre commander has 1000 HP, 10 DEF, and a set action rotation: Smash I - Attack - Smash I - Attack - Smash I - Windup. Note that the rotation advances on turns where the enemy can't act, including the surprise round if there is one. So if you attack the ogre commander from behind he will skip the first two turns of his rotation, and do Smash I - Attack - Smash I - Windup. Furthermore, this boss is accompanied by a goblin and a normal ogre.
To do the fight effectively it would be good to have enough magic attack to be certain you can kill a goblin with fire and enough attack to be certain you can kill an ogre with a critical hit. If you have both then you can be sure you'll kill both the smaller enemies before anyone gets to attack. Eliminating the ogre quickly is key, since he will use smash just like the commander and the debuffs will stack and refresh each other.
If you want to beat the ogre commander without removing Aura's collar, a good strategy involves using stasis bombs or Thunderbolt to disrupt the boss's combo. Turn order is hardly an issue given the ogre commander has only 7 AGI. With high enough ATK, or by debuffing the boss's DEF, Rampage is a good finisher move for the fight if you've got tactical advantage.