Equip any decent sniper rifle (powerful and accurate) with High Explosive Rounds (highest level preferably). This combination will kill nearly every normal enemy you hit directly and severely damages enemies in their proximity, if not killing them as well. For fun, if you shoot the enemy's feet or the ground he is standing directly on top of, he will jettison into the air. Note: High Explosive Rounds work best only with sniper rifles since they are meant to kill with one shot. If it is used with a pistol, assault rifle, or shotgun, the weapon will overheat in around five shots (depending on the type of weapon), regardless of upgrades and decency of the gun. This will make yourself vulnerable to enemies.