This is a guide that shows how to effectively land headshots using the L85A1, which is the second Medic unlock. The player in this video has the gun set to automatic fire, but is actually taking quick singular shots at the enemy. The L85A1 is an interesting gun in that its accuracy quickly dimishes, but on the first shot it is incredibly accurate. Thus, the trick is to calm down and take singular shots, followed by a burst to finish them off. Try it in game.
Here is another guide by Hakei that is showing how to use repeated clicks to hit enemy attack helicopters. However, what is most interesting is that it shows how the hitboxes lag behind the helicopter model in game. Thus, when aiming with the TV missle, just before it hits move to where the Helicopter just was. Watch the video to see more.