Ranger – “Emerald” [5th campaign, 2nd mission]

By the way, there's a trick. The northeastern castle doesn't have a shipyard; initially, there's only one ship, and a small army arrives on it once, which Fedya easily defeats. If the ship is preserved, I would put a supporting hero with a minimal army on it and send it to explore the seas and scout the map. By the time the supporting hero arrives, I would have cleared the island and taken control of all the mines on it. Then, Fedya would go down into the dungeon and move towards the central castle in the northwest.
In general, as long as the supporting hero doesn't leave the ship, and Fedya calmly occupies the demon castles, the enemy heroes stay quietly on their shore and don't move...