It doesn't seem too difficult here... Although I failed twice myself.
Basically, you attack the undead with Markel, and the humans with Godric, and the elves – whichever you choose. If I'm not mixing things up, there are a hundred skeletons on the right (a bit of a walk), about 18 vampires on the lower left, and 18 liches on the upper left, right next to the castle... I initially went after the titans (on the very first day)... I killed the titans, but lost a lot of troops, and then I didn't use my liches efficiently... Markel should have Ice Bolt and Raise Dead. So you can resurrect your own troops. The most important thing is to attack everyone with full mana.

You've all got it wrong :) On the island with your starting castle, or rather in the southeast, there are guarded "death knights" that will join you. It's better not to attack the vampires, as you can lose a lot of troops. Instead, attack the death knights guarding them (northeast of the castle, on a small island). The skeletons will join you on the main island, where you need to transport your main heroes.
On the right on that island (the large northern island), 51 death knights joined me (on black horses :D ):eek: :eek: :eek:.

Regarding the walkthrough:
nothing scary, it's all simple!
Godric only gathered resources, and I, of course, played as the necromancer.
I think I've described the most important points... If anything is unclear, ask, and I'll answer.

P.S. I just like them (the necromancers) :)

Sincerely, VDV_forever