Knock them out the moment they wake up, once knock out at night you’ll have all the time to starve it. Bring a high level high health Ovis if you don’t have the required kibble to tame it. Once the hunger drops at a certain level slaughter the Ovis and harvesting its corpse with a pick, transfer all the mutton into the megalo’s inventory and you’ll have an instant tame megalosaurus. I tried this and it worked!