To get this playstyle, you have to get precision headshots using a sniper rifle. You have to hold down the right trigger just a little to steady your aim before shooting or it may not count. It is possible that you may have to get five kills without reloading. It may make it significantly easier if you use an extended clip on the Kazo Sniper Rifle for this.
It is very possible to get this with the standard loadout on the mission Absolution. You start with a sniper rifle and you just have to time it right so that the guards below don't see you while you are sniping.
