I edited.
I think I sent Ike, Shinnon, Gatrie, Oscar, and Titania on their exploration party.
Which leaves 11 to defend. Brom, Boyd, and Mia (west stairs) defended the stairs. Har took Boyd's spot on occasion, but I also used him to swoop down kill people and come back up. Nephene, Soren, Rolf, Ilyana, Ranulf, Heather guarded the edges. Of course Heather and Ranulf only block the edges and can't really attack back well, but that is all I needed so I stuck them in the areas they were least likely to get attacked.
Myst did the healing, and of course Reyson gave units second moves which helped a lot.