The easiest place to get this achievement is during the mission; "Headshot", which is the third mission of the campaign.
From the start of the mission you'll be in control of a sniper class unit, you also have 2 other sniper units available to use.
Chances are this will be the first achievement you get in the game, if not it'll be among the first, it's just that easy. The entire mission consists of you having to use these snipers to clear out different locations around the map from the rooftops you start on, the first objective will have 5 enemies spawn around the opposite building to the one you start on, just be patient and don't take a shot unless you're sure you'll get the kill.
When going for this you must kill all 4 enemies with the same sniper unit, switching to another sniper will cause the counter to reset.
Lastly, the sniper magazine holds 5 bullets so you can miss a shot but at no point when going for this achievement should you reload, doing so will cause the counter to reset.