Do not fight the alpha of these if they are above level 20. I was patrolling mm the waters around my base when a level 45 alpha mosasaur attacked me. Within moments all 4 of my pliesaurs (levels 60-70) were dead, along with my 6 sharks (levels 45-125), my mosasaur (level 120) lasted a little while longer before falling valiantly. Just me and 3 turtles (level 60, 65 and 80) now. The alpha tanked over 400 hits from my blue crossbow (300 Stone bolts and 100 tranquil, i switched to tranq when i ran out of stone) before the turtles died. I died almost immediately afterwards. Seriously. The alpha of these things are just gross. I've now decided to quit the game.
I recommend a trap. Make it out of foundations, pillars, ceilings and a behemoth gate all in stone. Use an icthy with levelled speed and health to lure it in and close the gate behind it. Then simply tranq arrow it until it’s down. Preferably use at least an apprentice crossbow, prim will take way to long
Tame some use less creatures and feed to mosa and stay relaxed and feel free to knock it out
WARNING KEEP ALL CREATURE S IN PASSIVE AND ALSO HAVE A GET A WAY RIDE IF IT AGRO ON YOU OR TRYS TO KILL YOU AFTER KILLING ALL OTHERS
I PREFER BASILO FOR THE USELESS PART CAUSE THEY HAVE MORE HEALTH AND ITS PASSIVE TAME if you want any creature is ok