Oh, for the Necromancers it really couldn't be any simpler. As for development, it doesn't matter in any mission when playing as Necromancers because most units that happen to be undead on the map join you, strangely enough. Regarding this particular mission, from what I understand, it's the one where you need to get the daughter of some hero who opposes Catherine (the Empress basically, I don't remember her name anymore) out of the Tower.
Here is how I did it, though it' same a bit wrong, simply because I realized too late what and how to do... I only developed the Necromancer castle, searched everywhere and everything, then handed the army over to the Empress (she has a dimensional door), portaled to that island, took out the guy in the castle, portaled back, entered the Tower, freed the weirdo, and led her to the Necromancer castle.
But I think the correct way is this: you gather troops, and when you're ready—but still within the time limit—you take the girl and lead her to the Necromancer castle; meanwhile, that hero should come out of the castle and portal to you, you knock him out, bring the girl to the castle, and mission complete :)