It's actually quite easy to tame, the difficulty for me is actually 4/10.

Find a mosa, usually spawns around Carno Island (Aka The Dead Island), if you can't find one, tidy save but if not kill some leeds and plesios because these are actually the creatures that take the points for a mosa spawn.

Find an nearby ammonite and attack it and then get the mosa to aggro on you and go up to the shallow waters. After the ammonite rage has ended the mosa will still follow you for a while bit eventually it'll stop chasing you and will be stuck due to it passing its barrier. Shoot tranqulizers at it and when its' knocked out feed it narotics/bio toxins and feed it its taming foods (Recommended using a sooting balm and its preferred kibble) . Wait for a while and you'll get it! Goodluck!