Easy way to tame on mobile (without a trap):

-Get a high lvl ichthyosaurs

-Lvl up movement speed

-Get scuba gear, a good crossbow, and a lot of tranqs

-Shoot at the mosa while staying on your ichthyosaurs

-Every time the mosa gets close, just swim and it won’t hit you

The good thing about using an ichthy is that nothing will agro on you besides the mosa

You can also use the same technique on basically any knock out tame in the ocean