When you need to run a long way and you have some objects with you (for example, a non-explosive barrel), if you have the gravity gun, use it as cover and run (while facing the enemy) to the other side. Then if possible, shoot the barrel at the enemy. Note: Make sure there are no more that one enemy while doing this. Also, make sure that the object is indestructible, and most importantly, look at the enemy and see if you can kill it with one shot.