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
Besides the giga this is the hardest dinosaur to tame, note you will need a lot of narcotics and food. Before you begin the tame I recommend at least 2 to 3 full scuba suits for obvious reasons. You need to have at least a Ichthyosaurs or Megalodon, so you can actually move around and find a decent one. Once you find one you want to tame simply start shooting at it with the crossbow. Now right when you get his attention swim up, you should notice the Mosasaurs stops following you after a certain height. The Mosasurs cannot go higher than wherever it stopped. So simply park your water Dino above it and keep shooting it. If it runs off keep doing the same thing over and over again. Sooner or later you will take him down. Good job! Then simply use this app to figure out how many narcotics/food you will need to tame it. Once it is done enjoy your king of the water.