Well, I just beat Karador on Hard, with a mage and an all - well, mostly all - undead army, without losses, by round 11.
(Of course, I was playing with a mod that gives me a spell "Animated dead" which lets your resurrect fallen undead, as the Necro Call spell should work in the first place, but that's barely cheating - it costs the same amount of mana as Resurrection, only it's Chaos magic).
See the screenshots - I was level 27, 12 Attack, 17 Defense, 32 Intellect, 172 Mana, 16015 Leadership. I used several items (Dead Skull, Archimage Staff, Mantle of the Wizard, Old Skull, Royal Snake Ring, Sorcery Picture to boost my mana, and the Silver Chain to protect my troops from undead. I had Chaos and Distortion maxed out (Order is quite useless for me as most of its spells do not work on undead, not even God Armour). My wife was, of course, Zombie Rina, and I had Dark Commander maxed.
My troops (see pictures):
-10 Green Dragons (normally I use Bone Dragons, but their poison attack is not great against undead. Plus the mana stealing really helps here).
-88 Necromancers (great ranged attack, enemy debuff and of course, for Necro Calling enemy troops)
-87 Ancient Vampires (in this battle, mostly tanking in regular form as life stealing doesn't work)
-105 Black Knights (my main tanks, very resilient and can deal good damage as well once they get into 'swing' thanks to Increasing Anger. Not to mention raising undead morale.)
-123 Cursed Ghosts (while they cannot use their ability here, the 50% physical resistance+silver chain makes them very durable in this battle)
Karador's troops:
-6 Bone Dragons
-102 Necromancers
-50 Ancient Vampires
-116 Black Knights
-92 Cursed Ghosts
-111 Ghosts
-474 Zombies
-654 Skeleton Warriors
-881 Skeleton Archers
(Little note - did you know Karador's Zombies and Black Knights have a speed of 3 instead of 2?)
I used a little trick - with the help of the Crown of Blackthorn - to drink from the fountain of rage next to the Book of Death in the Great Forest and then run all the way to Karador, thus starting with 46 Rage.
1. round - Two well-placed Fire Rains greatly decimated the ranks of Karador's troops in the middle, including both stacks of necromancers. I followed up with a Black Hole, killing off the Bone Dragons (that is important). My necros fired into the crowd, decimating the Skeleton Archers too. My cursed ghosts took out the front plank barrier, then waited... Predictably, Karador used his magic to raise the bone dragons again, then his ghosts took out the cursed cross. His Black Knights charged in, attacking my ghosts - and blocking the entry to my troops thankfully. I quickly ganged upon that troop, then had my dragons fly out behind them and use mana source to great effect (20 mana back) on the ghosts and knights.
2. round - One more fire rain to kill off more troops for Karador to raise, then I blinded the zombies - they should not be underestimated. I then used my dragons as bait and killed off his vampires, parking them in front of his right side necros. The ghosts took the bait, and my other units ganged up on the knights.
In the following rounds, Karador only cast Necro Call (I always left him some troops to raise) or for some reason, Slow, while the dragons and necros killed off his smaller troops, and I used Doom to quickly finish off his knights in one round. I then used Pygmy to kill off the zombies - raising his necros with mine to prevent more troop raising then needed. Of course, I had to use Lina's chargers once to re-fill my mana.
By round 9, all of Karador's troops were dead(er). I simply raised the too dangerous ones and parked my units on the rest. I used my mana to raise the few units I have lost (4 knights, around 30 ghosts, 1-2 vampires).Then my knights killed off the crystal in 2 rounds... goodbye Karador!
Frankly, I had a much easier time here then with my mixed-army Paladin. I think the fact that this battlefield is considered a cemetery, giving +1 morale to all my troops, also helped.