I had my hero capture the nearest castle and build Hydras there. Railag, in the first week, would conquer what he could without significant losses, and at the beginning of the next week, he would buy reinforcements and head through the portal south of the city, running towards the city in the center. I don't remember the exact location.
With two Hydras, Shadia could capture the second castle on the last day of the week without losses, buy reinforcements, and then retreat to catch up with Railag.
On Veteran difficulty, you don't need to run anywhere. Shadia can defend against all attacks from the north, while Railag leads the offensive in the south.