Playing as the Necropolis faction, everything is straightforward. As for development, it doesn't matter much in any mission, especially when playing as the Necropolis, because most of the undead units on the map somehow join your side. As for this specific mission, as I understand it, it's the one where you have to rescue the daughter of some hero from a tower, who is an enemy of Catherine (the Empress, I don't remember her name).
Here's how I completed it, although it might not be the optimal way; I just realized what to do a bit late... I only developed the Necropolis Castle, explored everywhere, and then transferred the army to the Empress (she has a portal). I used the portal to teleport to that island, cleared out the area around the castle, teleported back, entered the tower, rescued the girl, and escorted her to the Necropolis Castle.
The correct way is probably this: gather your troops, and when you're ready, complete the mission within the time limit by rescuing the girl and escorting her to the Necropolis Castle. At the same time, the hero should emerge from his castle and teleport to you. You defeat him, escort the girl to the castle, and the mission is complete.