Successfully complete the game on the Normal or Hard difficulty to unlock the Inhuman difficulty.
An easy way to kill the giant Tyrannosaur is to quickly run to the giant tree in the middle of the nest site. Make sure you have a shotgun, explosive tip arrows, and grenades. Once in the tree, shoot a flare so that the dinosaur's head is in your line of fire. While she is looking at the flare, shoot as many explosive tip arrows as possible at or around the eye area. When she jams her head in the trunk to eat you, shoot her with the shotgun until she retreats. Once she frees her head, throw grenades. Repeat this until she falls. Then, quickly run over to her, and kill her with a knife through the eye.