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.