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

Well, this mission is definitely tough and challenging compared to others.

It depends. If Faidaen is well-developed and doesn't lag too much, it can be completed quite easily. I developed Faidaen 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, Faidaen 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 necromantic troops. And if he has the Raise Dead skill, which he should have by now, he can advance without any losses. By the way, it's very convenient to protect native archers with ghosts.
Another significant bonus is Talannar, with his super ability. He also fights very well with necromantic troops, thanks to his constant luck. I even considered sending him to fight all the enemies while Faidaen leveled up on the training grounds.
The first necromantic castle can be captured in the first week. However, there's not much point in doing so, as there aren't enough troops yet to go to the Dungeon. Therefore, the first two weeks can be spent defending the mines on your island, and then you can go and 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, and it's difficult to fight them without troops.
In the Dungeon, the route is approximately as follows: first, capture the castle in the center, then go back and down. Along the way, collect two dwarven artifacts and take a bow from the dogs, which provides shooting without penalties. Capture the lower castle and advance east and north. Collect two more dwarven artifacts and emerge on the surface near the central necromantic castle.
And then, depending on the situation. If you advanced quickly, the enemy won't have that many troops. Neither Lucretia nor Vlad posed any particular problems, and Nicholas doesn't attack. Defeat all the black dragons, they are usually near the castles. And then Nicholas.