In order to get a S Rank, you must play the game on either Standard or Hardcore difficulty. Assisted will not unlock you the achievement. The requirements are as follows:
A Story - 3:30:00 for S Rank
B Story - 3:00:00 for S Rank
You need to learn everything you possibly can about the game. Learn the locations of key items and how to progress fastest. Know the positions of healing items, ammo and grenades. Avoid damage from regular zombies to save healing items for fights against G. SAVE YOUR GRENADE LAUNCHER ROUNDS!! Lickers and G are the only things I recommend ever using them on.
Obviously, time is very important here and I'd recommend starting over if you don't exit the RPD within 45 minutes or so. The RPD is the longest part of the game for me so try and learn it to do it fast. Luckily, its possible to beat the game in less than 2 hours so the time we need to beat is generous.
Good luck!