Ranger - "Higher Vampire" [5th Campaign, 5th Mission]

It depends. If Faidaen is well-developed and doesn't lag too much, it can be completed quite easily. I developed Fedor focusing on strength, attack, defense, luck, logistics, and war machines. As a result, he moved quickly and dealt significant damage. The supply cart increased the attack of the archers, the enchanted ballista also provided good support, and during the castle assault, one tower was destroyed in the first turn by a catapult. Plus, by this time, Fedor already has a portal to the city and the ability to call in reinforcements. And another important point is that Faidaen fights very well with Necromancer troops. And if he has Raise Dead, which he should already have, then he advances without any losses. By the way, it's very convenient to cover the native archers with ghosts.
Another significant bonus is Talannar, with his super ability. He also fights very well with Necromancer troops, thanks to his constant luck. I even considered sending him to fight all the enemies while developing Fedor on the upgrade locations.
The first Necromancer castle can be captured in the first week. However, there is no point in doing this, as there are not enough troops at this point to go to the Dungeon. Therefore, the first two weeks can be spent on capturing mines on your island, and then you can go to take the castle. You can time it so that you capture the castle at the end of the second week and enter the Dungeon at the beginning of the third. Then you can call in reinforcements. Otherwise, there are about fifteen hundred dryads there, and it's difficult to fight them without troops.
In the Dungeon, the route is approximately as follows: first, we capture the castle in the center, then we go back and down. Along the way, we pick up two Dwarf artifacts and take a bow from the dogs, which provides shooting without penalty. We capture the lower castle and advance east and north. We pick up two more Dwarf artifacts and emerge on the surface near the central Necromancer castle.
And then, depending on the situation. If you advanced quickly, the enemy doesn't have that many troops. Neither Lucretia nor Vlad posed any particular problems, and Nicholas doesn't attack. We defeat all the black dragons, they are usually near the castles. And then Nicholas.