To kill an enemy quickly and easily, take the shotgun and hit an enemy like headcrab, zombies, and City 17 guards. Aim and hit it in the head to kill it. Additionally, you can use Dog's ball to kill; or distract and attract enemies to the ball for easy kills. When you first get the anti-gravity gun while you play fetch with "Dog", make sure to keep the ball. The ball is actually a "rollermine". You can keep the ball up until you go over to Dr. Kliener's fortified graveyard. The rollermine (ball) will follow you almost everywhere and is harmless to you, but it actually kills both types of Head Crabs. Other enemies such as zombies will ignore you and swing violently at the ball, giving you plenty of time to kill them, even with the crowbar. This will save you lots of ammunition throughout the zombie levels. You must be careful not to explode the ball, or it will be gone. If this happens, just re-load from where you had it last. The ball will follow you up most stairs, but you must carry it with the gravity gun up ladders.