Since they tend to retreat and charge after taking some damage,having a fast water mount (or at least flippers) is good to get off and back on when trying to ko or kill. I prefer the Sarco for its AoE bite against multiple and infinite stamina in water (however it only has one speed in water anyways, but it's still fairly quick depending on level). If escape is necessary bite it with the Sarco when it gets too close and swim away when the angler retreats. Crossbows (and most, if not every, other weapon) do work on its back, if you'd rather not get off.
Pro tip: This guy is not very nice, according to the encounter Nemo had with it, it did not end very well. If you encounter one just swim away and observe from a safe distance. When killed they drop angler gel which have their certain uses, its best to fight these with at least a Megalodon.