Here's another question – guys, has anyone tried completing a mission entirely with undead troops (after capturing their first castle)? Wouldn't that work?

In my opinion, it's pointless because:
1. There are 2 Ranger castles, meaning that to have more undead troops, you need to capture at least 3 of their castles.
2. Even if you capture them, you still need to rebuild them and accumulate troops, which requires a lot of resources and time.
3. I don't know about others, but I upgrade Leadership for Rangers, and it won't work with undead troops, which is also a big disadvantage.

Overall, there are only disadvantages, and I don't see any advantages. Unless you buy an undead hero and level him up on neutral creatures, while gathering a bunch of skeletons... But as far as I remember, there aren't many opportunities to level him up (I mean to a high level and with a large number of skeletons). So, if you use him, it's only to weaken enemies.

But in my opinion, it's not necessary; you can manage with Fedor. However, you might have an unlucky build, so you'll have to come up with something or replay the campaign. Personally, with the following build: Expert Leadership (I don't remember the secondary skills, but they aren't important), Expert Luck - Soldier's Luck - Elven Luck - Magic Resistance, Expert Light Magic - Mass Blessing (or whatever it's called, the one with speed and retaliatory strike), Expert Logistics - Native Lands, I didn't experience any major problems. The main thing is to move quickly, try not to lose troops on neutral creatures (or enemy heroes), rebuild your castles as quickly as possible (at the expense of rebuilding the necromancer castles), and actively use Summon Troops.