Instructions
Select the weapons you prefer to use prior to joining an online game server. The weapons you use depend on what weapons you've unlocked through the course of past games and dictate how you will approach your goal of killing opponents. If you prefer stealthy operations, pick a sniper rifle. If you want to run through the map and kill as many opponents as you can, a heavy machine gun will work.
Adopt an in-game strategy that falls in line with your weapons choices. For example, it's difficult to be successful while running around the battlefield with a sniper rifle, just as it's challenging to snipe opponents from a rooftop with a submachine gun. If you have a sniper rifle, find an abandoned building or a vantage point on a hill and lie in the prone position.
Shoot at an opponent whenever you see him. If you're close to him, aim for his head, as a head shot will kill him quickly. Otherwise, aim for his center mass and don't stop firing until he's down or has moved out of sight. If you're on the ground, pursue him. If you're in a sniping spot, change locations as he's probably seen you and will now be hunting you.
Use explosives, when possible, to kill opponents in large groups. Throw a grenade into a building inhabited by opponents or, depending on the version of "Call of Duty" you're playing, set up a C4 trap or Claymore mine near the entrance of a building.