The Mosasaurus is a deepsea creature, that means you can easily knock him out with this Trick. (you should use a basilosaurus because of his health and the immunity effects)
1) Swimm down to the deepsea
2) Let the Mosa follow you up
3) If you swimm high enought he will stop chasing you
4) When he Stops chasing you, just shoot at him with tranqs (the higher your Crossbow damage the faster he gets knocked)
5) Repeat the first steps if he swimms too far away from you
6) knock him out and feed him some Quetzal Kibbel
7) Enjoy your Aquatic Giant
BONUS TIPP: He spawn around the carno Island and i would recommend to not go too far from your basilosaurus while shooting the Mosa Because you are f*cked if some eals appear and you cant mount your Basilo to kill them (He is immune to the Shocking effect)
JUST TAME A SQUID GRAB THE MOSA,DISMOUNT THE SQUID.....and put the Squid on passive I will no let go of the mosa.AND THEN THE SQUID WILL BE DOING TORPOR AND ALSO YOU CAN HARPOON IT OUT AND TAAAAAAAAAAMMMMMMMMMMMMMME!!!!
Center tip: as they have added the mosa to the center I've found the easiest way to knock one out is find one close to the bubbles get its Agro and swim in the bubble the mosa will get stuck and it can't hit you. Also this gives you unlimited oxygen and sanctuary from being attacked by anything else. ðŸ¤˜ðŸ¼
Honestly this Dino isn’t that hard to tame, just make sure you bring a couple hundred narcotics and a fast swimmer, like a sarco. I would definitely recommend scuba gear, but as long as your oxygen is pretty high or you have a fast swimmer you should be completely fine. Good luck taming!