What you should do first is lean your body towards the edge of the wall and poke your head out from behind the wall (make sure your legs aren't showing)  and see where your opponent is.  This manuveur should take less than a second!  Now back behind the safety of the wall remember where your opponent is and raise your gun to the correct height in order to shoot him.  Then quickly strafe out from the side of the wall, make really quick micro adjustments (since you already know where he is you should have your sights on him as soon as you strafe out)  and take a shot then jump back behind the wall.  Now this is the tricky part.  Count a random number of seconds and take another shot.  Then count another random amount of seconds before taking another shot.  This is to prevent your enemy from predicting your movements.  Lastly, expect the enemy to move around so don't be caught off guard.  Good luck!