Two major ways to tame.

1. Build a trap. They can only fit through behemoth dino gateways. They cannot fit through normal doorways. Lead it (via flippers or an Ichthy) through the behemoth and then exit via the smaller normal doorways. Close the gate and knock it out. Megs can't damage stone, FYI.

2. Get flippers or an Ichthy and find a good shark. Shoot at it. When it (and all nearby sharks) gets close swim away. Repeat until it's unconscious. Put your food on it and then kite the other nearby megs out of render distance. They won't attack it while unconscious but they will as soon as it's tamed.