Equip any good sniper rifle (powerful and accurate) with High Explosive Rounds (highest level preferably). This combination will kill nearly every normal enemy it directly hits, and severely damage enemies in their proximity, if not killing them as well. For fun, if you shoot at 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, as they are meant to kill with one shot. If it is used with a pistol, assault rifle, or shotgun, the weapon will overheat in less than five shots (depending on the type of weapon), regardless of upgrades and decency of the gun. This will make yourself vulnerable to enemies.