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 (or it flies in the case of the dragon).
====Comands====
To straight up get the dragon:
summon Dragon_Character_BP_Boss_C
For all the items to get to the easy mode arena (less commands to put in):
giveitemnum 148 1 0 0
giveitemnum 150 1 0 0
giveitemnum 153 1 0 0
giveitemnum 155 1 0 0
giveitemnum 156 1 0 0
giveitemnum 159 8 0 0
giveitemnum 160 4 0 0
giveitemnum 158 4 0 0
giveitemnum 161 2 0 0
giveitem "Blueprint'/Game/PrimalEarth/CoreBlueprints/Resources/PrimalItemResource_ApexDrop_Tuso.PrimalItemResource_ApexDrop_Tuso'" 2 0 0