Use the knife instead of guns when fighting most dinosaurs. This is because the knife is an instant kill and will save you precious ammo in the long run. However, for Dilophosaurs and giant insects, 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, although you will die frequently unless you can get the timing done precisely. Since the game checkpoints before most fights, it should not matter.