Because you only have a small amount of bullets, you want to make sure they do as much damage as possible. There are a couple of ways to do this. You can move your enemies so two or more of them are around the red barrel. When you shoot this, it'll do damage to anyone in the adjacent squares. You can also shoot through enemies. If there are two bad guys in a line, shoot the one at the back and the bullet will travel through the bandit in front, killing them both.