I thought this was pretty easy using the Coral Lakes East's "Anomaly" boss mission (last in the list, I believe).
Be sure you have a wingman with you. There are three phases to the boss mission. During phase 1 use your main skill to pop all the balloons on the dragon's back. During Phase 2, use your wingman special attack (press X). Then during phase 3 do the same thing and just use your last wingman attack. The first phase takes less than a minute (about 50 seconds), and the 2nd and 3rd phase can take less than 10 seconds if you make sure your wingman attacks have a target. The total time to get S Rank takes about 2.5 minutes (with cutscenes).
Update: Video added. Overall this could take you less than 15 minutes to achieve.
There are some prerequisites to this mission. You need to be far enough along in the storyline, you need to have 3 specific antibodies, and you need 400 credits. I met all the requirements without trying, so it shouldn't be that hard. I was also on Casual difficulty, but you may want to try it on classic if you can.
Let me know in the comments if you have anything else to add. Thanks!