To survive, there are some basic rules you must observe:
-Rule 1:Normally, clear each area before passing to the next one or you will be overwhelmed.

-Rule 2:If you are unnoticed sneak and kill with a head shot and with a silenced weapon if possible.

-Rule 3:If you are REALY overpowerded or outnumberd,then RUN FOR YOUR LIFE.

-Rule 4:A head shot always kills.

-Rule 5:When facing a flak jacket goon, aimm for his head.

-Rule 6:Try to kill grenade tossing guys first, then snipers,then shotgun guys,then machine-gun enemies and then handgun guys

-Rule 7:Never get to close to an enemy

-Rule 8:Use the auto-aim with care as it may target enemies behin walls

-Rule 9:Choose well your weapons