Mission Rank S Achievement
Mission Rank S20
Awarded for achieving Total Mission Rank S
Getting rank S doesn’t mean that you need all S ranks. Just mostly S rank. You can check your rank on the mission select screen. Remember, in order to get S you need mostly S rank and the rest can be A rank. So try for at least A rank on all levels before going for this one. Also you need to complete the Hard difficulty as well as Normal on S/A rank.
See the achievement Mission Rank SS for videos on all the levels on Hard/S ranked.