Wait until the Bear stands up on its hind legs before you shoot. Darien will then aim at the chest of the bear with the game's targeting system. If you are not hitting it in the chest, you are wasting valuable ammo. The difficult part is getting the bear to stand up on its hind legs. You must stay close to the bear, but not too close. Do this by aiming at the bear and walking around it. If you get too far away, the bear will run at you. By staying close and walking around the bear, its attacks will miss. After you have killed one, you get to do it again. Just use the same technique, and the two bears will be dead in no time.