Approach the giant tree in the middle of the nest site with a shotgun, explosive tip arrows, and grenades in your inventory. Shoot a flare at the dinosaur with its head in your line of fire. While the dinosaur's attention is toward the flare, shoot its eyes with every explosive tip arrow you can. When she jams her head in the trunk to eat you, shoot it with the shotgun to make it retreat. Once it frees its head, throw grenades until it falls over. Then, immediately run over to her and finish her off with a knife through the eye.