The megapithecus has two attacks a direct hit attack and a ranged Boulder attack. The boulder attack can and will kill you off your tame and I figured this out the hard way.so safest thing is to wistle attack Target hope off and have someone roar the Yuri good luck survivors.
If you really want to go overboard bring in max dino cap (20 is Max) with at least 17 Rex's and 3 yutyranus. This will obviously increase your chances of killing it, try to level your Rex's in health way more than melee considering you have the yutyranus buff already assisting you or you can go full melee for a glass cannon run. Be aware that this may have a huge risk to it though if you don't have more than MINUMUM 10K health ON HATCH you may or may not die depending on your saddles and or melee damage. Don't forget the rock throw can kill you off of your mount so spread yourselves out if you aren't going to dismount. Otherwise whistle attack target and only mount the yutyranus for the empowerment of the Rex's. The rest of you bring compound bows, snipers, fab pistols, rocket launchers etc. You will also need some focal chilli to increase your chances of survival. Good luck :)