Manage to spend sometime ripping Valkyria Selvaria in action from Nico today :-
http://www.youtube.com/watch?v=EA2jBzy9P3U
http://www.youtube.com/watch?v=LTSvkPR9gqU
Apparently, you need to score S-Rank for all 3 stages in the DLC to unlock this secret mssion.