A rowdy beast is attacking cargo ships as they pass beneath some steep cliffs. Drive it away, and fast!!
This bounty is available to complete after after arriving in Hamelin.
Fluvius Maximus can be found in the ruins to the far south of Autumnia, near the beach you left the Sea Cow. It's got a fairly large amount of HP, but that doesn't necessarily make this a hefty battle. It resists water and is weak against fire, so a Burning Blade can make a nice difference. Keep launching All-Out attacks and the battle should be over before it has begun.
Defeat it and you'll complete the bounty hunt, return to the Swift Solutions to claim your reward.