Use your environment. Maximize the use of your surroundings, careful to make sure your cover is bulletproof, (wood, plaster, etc... only weakens bullets). Take advantage of those who use weak cover. For example: An enemy run up stairs, and know he's hiding in the corner? Try shooting up through the floor to kill him or distract him while your team mate goes up and finishes him.

Communicate. Communication is key to victory. Many Battles have been lost by players who fail to communicate. Use the voice feature and call enemy positions. (Be sure to keep an eye on the kill cam to comprimise your assasians position) 

Silent kills = More kills. Keep in mind, Players can always use silents kills such as the knife/melee kill to avoid showing up on radar. So if you work fast, you can take out more than one unsuspecting victim.

Customize your weapons. You will be able to fully customize your weapon load out to your play style. You can fully customize your primary and secondary weapons, add attachments such as sights, scopes, hand grips, or grenade launchers. Customize your perks to your liking. If you are a stealthy guy who loves to pull out a silent knife kill whenever possible, customize your perks with UAV Jammer so you don't show up on Enemy UAV Radar, or add a suppressor to your weapon so you don't show up when you fire.

Use your support by staying alive. If you stay alive long enough to get 3 kills, 5 kills, and 7 kills without dying, then you'll be able to utilize some helpful support from the sky. For example kill 3 enemies without dying and call up 30 seconds of UAV Recon (radar) for your entire team, continue your warpath to 5 kills to get the ability to call in an Airstrike anywhere on the map you choose, or even 7 kills without dying to radio in Helicopter support which will fly overhead taking down any enemies it sees and allowing some valuable distraction for you and your squad.

Learn the battlefield. Notice key markings on buildings so you can call it out to your team mates and work better as a squad. For example, if you see an enemy run into a small cafe, be sure to alert your team there's enemies in the small cafe to the north.

 Perks. You'll have Perks in multiplayer, which allow you special abilities, use them wisely based on the gametype /map your on. You can save up to 5 custom classes at a time, which you should use to safe a specialized class for each major gametype you play. For example if you're playing Team Deathmatch where kills mean victory then maybe take the Bandolier perk so you have extra ammo, or Stopping Power for increased bullet damage. If you're playing Domination or an objective gametype, maybe take Extreme Conditioning which gives you extended sprint so you can get the objective faster than the enemy. 

Always stay moving and in cover. Use your ability to Sprint to transverse wide open spaces and get from cover to cover, never stay in one spot too long especially after a kill because all enemies will be converging on your spot. Sprint is limited as you'll get tired, so use it wisely and to get out of a sticky situation.

Explosives are your friends.Taking an M203 or Claymores out to the fight with you can make a huge difference in Objective gametypes. If playing Headquarters, once you've taken over the HQ and are defending, set up Claymores around the major doorways to do the work for you while you watch your six. Or put some M203 rounds down range onto an objective to keep enemies from camping on it and capturing it.

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