Useful tip for everyone. When you need unique fodder, use the story girls!
-Lilium
-Alette
-Lisette
-Adelite
-Elena
-Brigitte

These girls are far more common than the other (in the silver recruits), as they can drop in both N and R (when the other can only drop as R). And I also think their droprate in the R category is much higher.

These girls are much more expendable. Use them as fodder EVERY time (or don't upgrade).