To tame one easily, build one pillar with a ceiling on top of it. Then agro the m3galosaur with now. When it gets close to you place a large hatch frame. This if done correctly will make the m3galosaur only able to run in circles for easy taming.
Tame the mega during the day! It is extremly slower during the day due to wanting a rest. It is a very strong dino, it has been tested to be even stronger than the allo! Be careful if you tame it at night ;) caw