Rogue Warrior

A good place to get this is after you blow up the bridge in Mission 4. Make sure you have an AK-74(AK-47). After you run through the burning hallways you will go up an elevator. Then you will come to a control room with a door on the left at the end. Use "LB" to hug the wall and open the door. There will be 2 enemies right in front of you and a 3rd above them on a staircase. Squeeze off some quick aimed bursts to kill them. Then enter the room and take cover behind the crate right in front of the door. 2 more soliders will come around the racks towards you. Use the remainder of your clip to kill them. If you Don't get all 5 in one clip, just reload your checkpoint and repeat. Tip: lean out with your LS from cover and use LT to focus your aim. This is actually more accurate than using your iron sights when not in cover.

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!!