Trying not to use Pre-Promotes yet cause I know they are EXP holes at this point, but probably should use them for defense stance since they'll provide pretty beefy stat boost. Mostly trying to get by with attack stance though because defense takes out a character and I've yet to find the benefits of it outweigh the loss off an attacker, especially since so few of my units really seem to have any good dmg on mounted so as many attacks as possible to cut down their numbers has felt like the way to go. But I'm guessing this is a map that really requires you to use defense a lot and only selectively use attack. I've tried a variety of units outside of Ryoma, Scarlet and Reina, but by turn 5 or 6 once all the mounted are in play I just start having units drop like flies even with two healers.
I even have rather solid supports, most units have C rank with the majority of others, numerous have B and A ranks with several characters as well.