You need to summon the boss at an obelisk or supply drop. It requires all 8 artifacts from the Island, 2 sauropod vertebrae, 4 argentavis talons, 4 rex arms, and 8 megalodon teeth. The bosses are super tough, it is highly recommended you do this on single player.
For single player you can use commands to spawn in the items to your inventory (or grab the artifacts as it is tied to another achievement). Put Player Damage at 999 and Dino Resistance to 999. Use the command "infinitestats" or "god" before you engage. Use a bow and shoot the boss for about 2 billion damage and get your achievement. You could punch it but minions will get in the way.