A couple days ago I tamed a level 6 Mosasaurus. I used about 50 tranquilizer arrows (some were missed shots). I found it between South Zone 1 and South Zone 2. I led it over to the eastern end of the giant peninsula at South Zone 2 because there weren’t any dangerous creatures there. I gave it a soothing balm (I did this on mobile). I was actually riding on my Sarcosuchus to knock it out because I didn’t have access to the full set of SCUBA gear. I hope this helps you get a Mosasaurus for yourself!