Make sure that you move your Rexs or creatures into the trees when you get teleported into the arena. This is to prevent your tames from falling into the hole in the arena, because if they fall they just die, and you lose the saddles and everything. Then you can go up on your yuty or whatever you are riding and you lead the Megapithicus to your tames, then let them go ham on the monkey. The Megapithicus is a glass cannon and anything g with a good saddle and a high health stat can easily destroy it. Just make sure that nothing falls into the death hole and you’ll be fine to actually fight the boss.
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.
To battle the megapithecus, I would suggest using a pack of allos,a yutyrannus, and a daeodon. The reason I pick these creatures are because that first of all, allos in a pack are dangerous, and one of them will be boosted as the alpha, and the other allos will also be stronger. Second, yutyrannus’s can scare away the minions the boss summons, and can make the creatures deal more damage. I also HIGHLY request a daeodon because they can heal your Dino’s, and itself durning battle, just make sure it has enough meat to last the battle. This method will work with any boss on the island DLC, not the overseer, but I used this method on the broodmother,megapithecus, and dragon, and it worked perfectly.