Let's try to be clear here.
First of all: to unlock this achievement you must play STORY MODE and obtain one S after the another in the same game. If you fail, you have to start the entire story all over (!!!).
If you replay through "Chapters" and get an S, that doesn't count towards the chievy, so stay focused and try to gain more points out of every fight... yes, because in the end, the rank you reach is BECAUSE OF YOUR POINTS!
To gain points a good method is "stealth kill" as much enemies as possible (ie: take them from behind using knife). Don't forget to use doorways and wistle (X or Y while in cover) to attrack them and then hit (X or Y when they're near). A good move is the slide (Y while running) and hide: if they fell, their memory about your presence resets, and you can lure them from a door for stealth kill. Also, bear in mind that the map in the upper right is a huge ally: enemies can see/hear you only if they are on it, and on the same level as you are... usually.
Hide is the key to victory here, yet, the best method to have points is CHAINING KILLS TOGETHER: the points sum and a multiplier boosts the final score by the number of enemies destroyed. Example: if you chain one enemy worth 1500 and another one for 3500, the grand total will be 10.000 points (1500+3500 x2)!!!
But this is not enough for the S. You also need to beat the time limit with no deaths. If you manage to finish each chapter under a certain time, you will get a x2 on your total score, and another x2 if you finish with no deaths! So it'll be easier for you to reach the S with a x4 up your sleeve!
The PAR for the first level is 50 minutes, for the second 100 mins and for the third is 70mins. Easy enough.
If you die, you can still have the final x2: just reload the checkpoint before the game respawns you (press START and select the option)! If it respawns you before you can hit START, just get back to Xbox dashboard, lunch the game again and press "continue" from the main menu...
I reccomend to restart also if you feel that a fight could give you more points. In the end, this is all that matters! Ulyssium is pretty useless here (sorry mutants!). ;-)