For this fight, you will need 19 rexes and one yutyrannus. Portal is situated at the Blue Obelisk Terminal
Artifact of the Brute: 530010
Basiliosaurus and scuba gear needed.
Follow the path to the left every turn, till you end up in a big water chamber with the artifact in the middle
Artifact of the Pack: 685560
Crossbow, grappling hooks, bug repellent, weapons (best place to find megalanias)
Stick to the left every time in the cave, until you approach a small drop with xp book at it. Always stick to the right after that.
Artifact of the Devourer: 140863
Tamed Barryonx, possibly scuba gear, weapons
Follow the path, until you have to go underwater. Swim to the left in the water, then to the right. You will then be led to a massive ravine cavern, with glowing blue crystals on the walls. The artifact is right below you on the very bottom floor. You literally cannot miss it because it is glowing a bright purple.
5 Tribute Items:
Thylacoleo hook claws
Once at the arena, BE CAREFULL AS HELL THAT YOU AND YOUR TAMES DO NOT, NOT, NOT FALL INTO THE HOLE AT THE RIGHT. YOU AND YOUR TAMES WILL DIE IF THEY FALL IN, OR YOU DO. Quicky get all your dinos in the fighting arena, as the boss has knock back damage. Fight at beta difficulty.
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.
Hardest part about this boss on The Island is getting the Spino sails for the alpha summoning. Spinos don't like to spawn on this map (FYI they are water spawns so kill fish to try to get them to spawn in) and I had to go to Crystal isles and transfer a few stacks over. He's a pushover and a glass cannon. Just avoid the pit falls in the arena and bring him to your tames in the trees and swarm him with theris or rexes while you ride a screaming yuti and he goes down really fast.
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.
Before entering the arena take your creatures out of their cryopods (if you use them) because you can't use cryopods in the fight. I found this out the hard way and almost lost all of my boss fighting Dino's. Fortunately if you need to escape the fight once it's started you can leave the game and rejoin and it will place you and your tames at the obelisk (doesn't give summon items back).
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.