When fighting most dinosaurs, use the knife instead of guns. The knife is an instant kill and you will save precious ammunition in the long run. The only exceptions for this are the Dilophosaurs and giant insects. For those, use some type of explosive or specific weapon to stun them or knock them off their feet for a moment before you knife them. Even under the Hard and Inhuman difficulty you should still use the knife on the dinosaurs. You will die frequently unless you can get the timing done precisely. Because the game checkpoints before most fights it should not matter.