Best way to Tame a Mosa or at least what I do is. . .

I hit an ammonite and with the agro effect the Mosa should follow me to the surface, once the effect of the ammonite passes the Mosa couldn't return to the water and be stuck on the surface and u can shoot him easy from the Coast

This works with other animals like plesi, dunkleo, etc, u can even kill Alpha mosa with this trick.