Board Thread:Unanswered Questions/@comment-26417223-20150821135519/@comment-26402553-20150824015153

You can only S rank a mission by reaching a specific score or go above it, there's no need to kill every single enemy before the emergency mission as long as you achieve the minimum score for an S rank. Still if you leave out targets in the first part of the mission the chance is very high that you won't achieve S rank due the minimum score not being met.