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Well, perhaps it's necessary to move faster, without lingering anywhere. Does Railag have logistics? If so, he will reach the garrison faster than the enemy heroes. Shadia shouldn't stray from him even a step. Railag, with proper development, will defeat anyone; Wayer's army isn't the strongest, and with proper management, even with a minimum number of hydras, you can try to win.
For a long game: hide from enemy heroes and gather troops in dwellings. They won't go near the Dungeon. Also, destroy a couple of castles. Then gather your forces and finish off the last one. No one will bother you anymore.
But if you're playing on "Hero" or "Veteran," my advice is to lower the difficulty. Then everything will be quite manageable. On "Recruit," I cleared the entire map.