You need a Plasma Pistol And Battle Rifle.
Hold down the right trigger with the plasma pistol. Point it at somone who you can see. Shoot. It will work whether long, medium or close range (although at long range it is best if the target is standing still. Straight after shooting the Plasma bolt press 'Y' to change to the battle rifle and quickly give them a burst to the head. They will be Dead.
The plasma bolt will take off all their shield and the battle rifle finishes them off.
The smoother you can get the change (between weapons) the better, practice makes perfect.