From StrategyWiki, the free strategy guide and walkthrough wiki

[edit] General Tactics

These tactics can be used in almost every type of gameplay:

• Watch the Radar - It can tell you where the enemies are, this gives you the chance to throw a grenade and prepare yourself or tell you which direction to go in.
• Call in UAV while Sprinting - This means that you don't have to wait for the animation so you can react more quickly to enemy attacks.
• Learn the Maps - One of the most important things to do, learn the hiding places and where to fire Airstrikes. This will help you a lot.
• Stay close to walls - This means the enemy will have more trouble trying to hit you and you will be more camouflaged.
• Move in the crouch position - This makes you less of a target and people can hear you less.
• When going into open spaces, sprint - This makes you less of a target and may show you where enemies are. Try not to do this for too long.
• Aim down your sights - This makes you a lot more accurate.
• Throw flashes inside closed spaces before you enter
• ALWAYS move to diffrent sniping spots every minute, your never safe

[edit] Free-for-all

• Stay on the move constantly - Campers are found and blown up rather quickly in this game mode.
• Learn to hip fire - SMG's are very efficient in FFA mode for this reason.