This is a tough little scenario - partly because of the huge density of enemies, especially long range ones, in the little building. You also have the great big creature (I think it's a "Grotesque", but I can't Anglicise this Japanese too well...), who doesn't move much, but is a nuisance.

The most pressing matter to begin with are the Brave Knights to the east and wst of the map - they charge straight away, so you'll need people with Phalanx there to stop them. They have Dragoons and archers, so it's important to protect your generals. At the beginning of Turn 3, ~Kurai-ru~ appears at the bottom - I don't know whether it's random or not, but sometimes he appears at the bottom left, and sometimes central bottom. If you can keep him pinned with archers, he won't cause you any problems at all.

Now comes the actual penetration into the building. The Priests do cast a lot of Attack 2 on the General at the front, but he never seems to do anything. Take him out with archers or your best foot soldiers. Beware of the Grotesque - it's very slow, but fires some kind of blast in the area, which damages everything, enemies included. The damage isn't huge, but it will hurt archers a lot. With the General out of the way, weaken it with archers before attacking it with your most powerful generals. Spells are a waste of time, and units will get obliterated.

Finally, you'll need to run the gauntlet of archers and spell-casters. My tactic was to cast a couple of powerful spells in there (two Tornados and a Thunderstorm), before charging. As most of the enemies are archers, actual combat won't be too damaging for you, and with the spells weakening the Ballistae, they won't be a threat. Even still, everyone will be casting Heals, so take them out quickly. And don't forget the secret tile to the north-east of the map.