Use Vanessa or Tana on turn one to lure the mercenary next to his starting location away from him. You can put Vanessa/Tana on a mountain to achieve this. This keeps Gerik in the building. On the following turn, the cavaliers will move in to attack him. He should retreat and use his vulnerary. Then, Innes will try attacking the cavaliers.
From there, Innes will get hit by one cavalier per turn until he takes enough damage to retreat and use his Vulnerary as well. After that, it should be tethys tanking hits which she can. She has 80 evade on enemies with about 90-110 accuracy, and she takes about 3 hits to get KOed. So, with method, those three can survive on their own for a long time. If you wan to help them out a bit, send a unit to the side of the building to distract the backed up fighter and cavalier units.
You want to keep those three alive. Tethys is wonderful. Gerik's hero crest is really need to get another unit promoted, and he makes a wonderful unit too. And the white gem isn't too bad either to avoid having to keep restarting for rune swords in the tower.
With those three fending for themselves for the first 10 turns or so, my first task was to clear out the 2-3 paladins and the two sages in my starting position. From there survive the other two paladins and enemies rushing my way. After that, I could enter the lower half of the map to trigger Marisa and her units to rush me. Retreating Marisa sprouts reinforcements to the mountains in the southwest, a few mounted units in the south east, and a couple enemies in the northwest starting mountains.
I guess the other strategy I was contemplating on this map was to use the physic staves I bought on the chapter 9 vendor to keep Gerik's health up. Although, I do not think this would work in the end because once he was out of the building, he would be surrounded by cavaliers and fighting the mercenary on his own.
