All there is to do is try kiting it to water to drown; hopefully there are sharks in there to help. Take a fast flyer, park in high places, when possible, to shoot it. This makes it keep chasing you. Don't let it bite you or your mount. Repeat and keep it in the water. It doesn't seem to be able to bite while submerged. Keep attacking with your mount when it's in the water so it wont turn back onto land. Best of luck.