Easiest way to earn this achievement is going thru heaven or hell difficulty.
Choose the basic costume, don't use super costume (it will penalize your score).
Use range weapons, Neros range weapon, dantes pistols.
I'm pretty sure the score for S rank in this mode is 1000. Very easy to get if you don't take damage and don't use continues.
On a side note, the game tracks your S rank completion across all difficulties. So if you only need a few S ranks, mop it up in heaven or hell.
Happy hunting :)