A huge turtle has made Skull Mountain its home. Best deal with it now, before it moves on.
This errand is available after defeating Candelabracadabra on The Tombstone Trail.
The Shellraiser can be found south of the Skull Mountain, which is on the northwest segment of the Summerlands continent. Just look at your map for its specific location, which is across the northern river. The creature is resistant against water and also has decent defense in general, so you might want to use some attack-boosting skills such as Mite's War Cry. Pummel away, stay healthy, and the turtle should fall sooner or later.
Defeat it and you'll complete the bounty hunt, return to the Swift Solutions to claim your reward.