Battlefield 3 Multiplayer tips of various levels from basic to more advanced.
Set traps – When you have C4, or even better if you have Claymore mines.
Place your trap in a place that has only 1 or 2 ways to get to you, and try to cover the entry. Then get irritating – shoot a bunch of enemies up (even if you miss) or even just fire around a lot until you draw attention. When someone comes, set off your C4 (you have to be able to monitor someone’s approach) or if you used a Claymore, its set-it-and-forget-it. Just wait to hear the BOOM.
Set a different trap (the “Lure”) – Get REAL annoying by setting up a mortar and shelling the enemy. If they know what they are doing, they’ll come after you. If your team knows what they are doing, they will cover entryways to your position and shoot the “fish in the barrel” as they approach. Get ready though, you will get killed because after enough time they will probably get through. You could also use the MAV, a loud weapon, or anything that would grate on your enemies’ nerves to set a trap.
You can also find a place where some of your teammates have holed up and maybe even camped, and lure enemy soldiers into their sights.
Be ready at all times – When you are running around, when you are sniping (in the scope), when you are sitting still in that dark building where you can’t be seen, wherever you are you should be ready to start shooting at a second’s notices. First and foremost that means always reloading after you have fired your weapon, especially if you get close to empty. If your assault rifle is down to 5 bullets, you definitely need to be looking at where you can safely reload. If you have an M249 you might reload when you are in a “safe place” when you get down to 20-50 rounds (personal choice, of course).
So now you are in the habit (hopefully) of staying reloaded, now you need to be ready when an enemy soldier surprises you by appearing “out of nowhere” right in front of you. Right away you have to be aiming and firing (or firing then aiming, depending upon the situation). That means being ready to fire immediately when you run into someone face-to-face at short distance or when someone runs right in front of where you happen to be sitting/standing/camping.