When taming a mosa on mobile, I recommend that use use a soothing balm of any type, them use and prime meat or normal meat (not fish) the tame and ithy, they dont agro to them so you can use a cross bow and tranc arrows to knock it out while riding it. Make sure to clear the area first before putting th stuff on it , and dont forget to bring Scuba gear.

If you've been shooting this thing with tranq arrow and then you need to go back to your base for something before you knocked out it, give up. This thing torpor drain rate is extremely high, missed shot is already a big mistake. You'll end up killing the mosa instead of knocking it out. Sarco is highly recomended for water mount, bringing army such as basilo is a stupid idea, mosa is the strongest water creature, your basilo won't survive. Use the pokeball thing to bring your basilo, i know mosas territory is so damn dangerous not because of the mosas, but the electric eels and jellyfish, also if your sarco has 500% damage or more, you could kill plesio and alpha shark without killing your mount, atleast in one vs one battle, not one vs herd.

Up the arrow right now, this is a critical information for beginners.

Unconventional taming method of the day

If you have a tuso and find one near herbivore island or another place with a loch of some kind, use the tuso to force the mosa into the barricaded shallow water. The AI will spaz out completely and it will try to flee, but block the entrance with your tuso and it won't be able to escape.

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!