You have eight missions to complete. Each mission consists of a different location stage, and a different enemy boss unit.
You can choose which maniple to send on each mission, according to their strengths and weaknesses. For example, you will have to send one maniple to the Space location. The Saviour or Argent might be poor choices here due to the lack of cover, so you could send a faster maniple like the White One or Templar. If the boss unit in the Space location is the Banrigh, you might want to avoid using the Templar because of its low health.
I think this is quite nice mode. I usually play the All Random mode, but now you can avoid the randomness in favour of making strategic choices.