Do your best to avoid killing and alerting any enemies. Play smart and use the Wu Silent Pistol when necessary. Take your time, line up the reticle, and go for those headshots so baddies will fall asleep instantaneously.

You can finish all six missions on normal difficulty with an S-rank and earn the achievement, you don't have to worry about doing this on hard.

Upon clearing a mission you’ll come across a results screen. Certain records will be displayed for that mission depending on your performance. For the most part, most missions require that you garner 50000 points or higher as your final score before rewarding you with the S-rank. The only exceptions here are the "Intel Operative Rescue" Side Op and the "Jamais Vu" Extra Op, as these are more action oriented missions (approximately 20000 points should net you the S-rank provided that you don’t kill many people while you’re out of the chopper in Intel Operative Rescue). These points you earn in the results screen at the end of a mission do not accumulate for repeated playthroughs of a mission; the points are only there for that most recent playthrough. Here’s what to keep in mind:

1. Be quick and efficient. If you have to replay a mission, keep the objective locations fresh in your mind and get it done. Don’t mess around, stick to the objective. Memorize the areas where guards stay put.

2. Enemy combat alerts. If you tranquilize someone with the Wu Silent Pistol, and a patrolling guard catches one of his allies "sleeping on the job," you should still be able to continue just fine as long as you aren't spotted. If that white detection line fills up on your screen and you enter reflex mode, take care of that enemy quick and kill only if you need to. Don't enter reflex mode too often if you have it enabled!

3. Killing subtracts from the final score you earn in a mission. If you kill one guard, that’s minus 200 points from the final score (if you kill two, minus 400 and so on). It's best to just use the Wu Silent Pistol.

4. Rescue prisoners. For each one you save, you are rewarded with 3500 points for your final score in that mission.

5. Do not lose your helicopter when you call one in. If enemies see it, they will open fire at it. Select a safe spot that is secluded from enemy units’ line of sight. I usually stick to the cliffside east-northeast of the cages where Chico is located in the main mission when designating a landing zone.

6. If you make a small blunder such as accidentally wasting one tranquilizer round, it won’t be worth it to retry a mission (unless you’re that stubborn). Restarting from the last saved checkpoint subtracts 300 points from the final score.

7. Disable reflex to earn a point bonus in the final score (not required though, I got S-ranks in all missions on normal with reflex enabled). It must be disabled before you start a mission to count.

Maka has provided video solutions for the S-ranks, if you are having difficulty be sure to check out his videos with this link (click here). He provides some nice commentary that will guide you through each mission with ease.

If you'd rather not listen to commentary, but would opt for some quick solutions just to be done with this achievement, take a look at PowerPyx's S-rank solutions in his YouTube playlist (click here). However, this doesn't cover the "Jamais Vu" Extra Op on Xbox, but the PlayStation exclusive "Déjà Vu" Extra Op instead. You can view Maka's S-rank guide for "Jamais Vu" under the "Hidden" achievement down below.