ok, I just looked at maps and I remember that one now. What I typically do is have a small team (including Ike) leave the base on the eastern stairs and make their way down to the boss. They have to kill the boss on the end of turn 10 so I can get max experience all around.
The rest of the units defend the upper level. So you need 3 strong defenders and a handful of ranged units up there. Also, 1 healer, and a heron to give your healer multiple turns, or other units if need be. I stuck my armor on the west stairs, and Mia on the eastern stairs.