(Day time technique)...You can KO the Mega by keeping it awake, just bump into it with your caving Dino (don't attack) and lead it toward the mouth of the cave.

It will want to fall asleep multiple times, Continue to wake it up and it will eventually get Knocked out.

The benefit is that it's heath won't drop, so it allows you to tame higher lvl Megas with tranq arrows vs darts... for the second and sometimes third time you have to KO the beast. (Multiple KO required due to it waking up at 20:30 every night, they fall back asleep at around 5:15)

More Megalosaurus Taming & KO Tips