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)
Wow... OH hello! So these giant sea creatures have massive turning circles! You can stay in their blind spot and shoot at them with harpoon and tran bolts. If you must bring a dino, turtles turn to slow. Alligators
Turn to fast.
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!!!!
Use 5 behemoth stone Dino gates to make a trap and make sure you have a small space where your tame can escape. Then lead him in and close the gates. The just tranq the heck out of him and feed him kibble 👍