After obtaining the thermal scope for your sniper rifle, use it in bursts and to tag enemies. To tag, hold the crosshairs over an enemy. A "tagging enemy" meter will appear. Keep the enemy in your sights until it is filled. When an enemy is tagged, a symbol will appear over their heads. To do this effectively, activate your thermal scope, then quickly memorize all enemy locations. Turn off the thermal scope before it expires, then tag as many enemies as possible. This tactic is very useful in missions where you are tasked to kill a specific type of enemy, such as Combat Medics.