The easiest level to do this on is the Prologue due to there not being as many enemies and it's a relatively quick mission. Make sure you do this on your easy playthrough as it can be hard on a higher difficulty. Make sure to keep in cover and take your time, there is no point in rushing because it won't help you at all. Make use of your traps they will come in handy, your machine gun will also help a lot going for this achievement. If you get hit just restart checkpoint and it will still unlock.
Thats def a challenge on the highest difficulty i can imagine, so sounds more easy probly on the easier settings, but when its out if possible i try the hardest difficulty right away i love a challenge.
This is best done on cadet difficulty and on the prologue mission simply snipe the first target run out the run snipe the next two, sprint to the hole go prone and crawl through, ignore the damage from the tank and then hide behind the box straight ahead attract the guards outside with rocks and proceed to take them out with the Welrod next progress to the next building and stick behind the stairwell for cover and proceed to snipe the rest of the guards in the next building final hide behind the door once you pass the next room and snipe the two snipers visible then proceed to carefully attack the others then descend down the hill. You can use checkpoints as long as of course you haven't taken damage beforehand!
Definitely easy on 'Prologue' as QuantumCola said. You can use your mg instead of the welrod, the noise helps to attract enemies so you can spray them in the doorway with short bursts, you wont even need to reload. The hardest part is the last lot. Just stay low and take out the snipers first. Pop up to get enemy positions and pop down again, aim in the right direction, pop up, shoot them and pop down again. Repeat and rinse and head down the hill when they're all dead.
Mission 5: St. Olibartus Church I done this on, was playing on Marksman difficulty so wasn't extremely difficult but still enough of a challenge, just got to take it slow, be stealthy when possible & wait for the right shot, will take a few checkpoint reloads no doubt to get it but it's defo possible on this level.
Yeah video in #7 is basically what I did... glad I watched it before posting my method. Only thing is, it might be worth mentioning that at that part when you sneak past everyone, those two guys chatting by the truck can probably be lined up for the Double Dose achievement (kill 2 with 1 shot) if you go a little past the doorway and sneak directly behind them, so you might be able to kill 3 achievements with one stone, not just those 2. I say "probably" because I stealth-killed the other 3 guys before I lined up that shot, so I don't know whether leaving them alive will make you get noticed before you can line up the shot.