Step 1: Tactics
Well first of all it depends on your gun, what works for a Tr18 probably won't work for a Wrectangle, but here's a few general tactics, we'll get to more specialized tactics a bit later on.
-Never let your gun run out of ammo, as soon as there is cover reload.
-Work as a team, you can only shoot one person at a time, so if two or three of you attack one person at a time you have a higher chance of success.
-You are not Chuck Norris, do not even try, you will fail.
-Make your shots count, don't just fire off your gun randomly, aim.
-Don't charge a group of enemies, doing so will result in failure, or death, possibly both, unless your gun has a high ROF (Rate of fire) and a lot of ammo.
-If you are going to pick up ammo off the ground, do it when there are no enemies around.
-Ambush your enemy while he is reloading.
-Use all available cover to your advantage.