[Bonar]

You need to try different army composition. Use units with better hp/tough, get your leadership as high as possible. Fighting general Karador (Necromancer's castle) is quite challenging if not hard. In my game, I used Dwarf Canoneers, Griffin, Horsemen, Orc Shaman and Inquisitor to built up my rage. And at that time I got the spell that damage undead but heal the living (forgot the name), do that tree of death/life quest to get it, you should! It's very very USEFUL!

The fight is quite a pain since they liberally ressurect any fallen units (Karador himself have way more mana than you and he is not afraid to spent it all), this is why I use tough units. If you spot any item that can curse, destroy it immidieatly. Concentrate most of your attack on the castle heart (the black obelisk), it only retaliate by casting negative spell. I had my griffins solely attack it, only ressurect them twice to keep full number (more damage). Backed up by my canoneers and inquisitors. Don't waste mana on expensive spells since the fight is quite long, I kid you not. Don't use the spell I mentioned before if it cover the castle heart, it will heal it also. Use it on your troop if it get surrounded by multiple undead units (AoE damage to undead and they run 1 cell, AoE heal on the livings).

I think if you can get your hand on any dragons it's a good odd, especially red or black with their special ability. You could get their eggs from their nest, located southwest of the map in that area. Warning, you will fights several dragons guarding that place!