Preparation is key to not spending 10+ plus hours like I did the first time trying to tame. First setup behemoth gates each with doors in the shallow water as a trap. I'd say 5 to be safe but can be done with just 2. Allow space between behemoth gates big enough for a mosa to fit tightly between. Make sure you have a speargun or crossbow to kill coels and sabersalmon since they spawn and eat the fish swarm. Go ahead and trap your megachalon. You dont need a roof on your trap since the creature gets stuck in the doors. Here's where things change depending on solo tame or duo taming. You must be within a few blocks for it to maintain in taming. So if you have a partner, choose who will stay with the megachalon and who will be pulling swarms to the tame. Put poop in your last slot to maintain aggro of swarm. You most likely will need multiple swarms. If doing solo, get at least 4 swarms just out of agro of megachalon before starting and then kite each as needed to the tame.
They have patched the render range range so ignore any tips that say to leave render as it will no longer. Guess I’m stuck with the turtles I have, I’ve spent three hours trying to take one of these things and had it walk out of the water which reset it taming! It was at 80% that time. Never got it above 20 after that. What he for the saber tooth salmon that spawn to kill the swarm if they bite the turtle it resets to zero and you start over. Extremely frustrating way to tame a creature. I would like to beat whoever came up with this idea.
Lead it into a line of stone behemoth gates and close a gate behind it and it will be trapped or you make a square box of Dino gates and nothing can mess its taming up while it’s being tamed so up this!!!!!!!