I found the easiest place to get this was at the start of level 2.
First off, play on easy at will let you take your time, and not worry about taking damage.
Kill the first two guys on the level, the go up the ladder. From there, kill move the guy at the top and pick up his AK 74.
From here walk up to the guy on the right and take your time and headshot him. The rest of the enemies will now take cover, but as damage is so light on you, you can take your time and headshot the next 4 whilst they hide in cover.
The advantage with doing it here is that the enemies hide behind the boxes, where you can still see the heads - so you can take your time and line up the one bullet kills. Equally, as you have loads of ammo in a clip for 1 shot kills, it doesn't matter if you miss a few!
Good luck!!