Some people have reported hearing some unsettling singing in the foothills of Old Smoky... Go and investigate!
This errand is available after completing the main story.
Your target is just south of Old Smoky, so Travel there, then walk down the path a bit to find the Magmadiva. As with the other singing lava lady hunts this one is weak to water, and isn't very strong. Stay healed and blast her with your water tricks.
Defeat it and you'll complete the bounty hunt, return to the Swift Solutions to claim your reward.