Definitely in vest in a aquatic tame, to tank the damage these things produce.. My 1st experience almost killed our tribes highest leveled Megaldon, and the Xiphactinus was only Level: 140. With that being said, beware of these deadly foes.

P.S. Also try and use Harpoon Tranqs. The Xiphactinus has low health, so if you bring any tranq arrows, you'll most likely kill it before you get the chance to tame it.