Get past the part where you cross the river and move up the building killing the flying dinosaurs. Afterwards swim to stairs. When you walk up them, there will be spears and a big black rapters like the one in the Venatosaurus level. You will be killing the first one, Kill it, but do not kill the second one. It will eventually end up killing itself. Quickly make your way to the building that has a thing on the top filled with fire. The second one will probably be out by then. Get up to the thing full of fire. Light a spear on fire. Wait for a good opportunity, then throw the fire spear and hit the raptor. Remain out of sight until the raptor decides to go to the cave where the ammo drop is located. As soon as the raptor walks into the bushes, they will start burning because of the fire spear. The fire will kill the dinosaur. This also clears your path, as the bushes blocked the route to the lever and ammo drop.