Things to remember when fighting him:
-If you manage to completely kill one of his stacks, he will ALWAYS use Necro call on the stack. This is a GOOD thing. You'd rather have him resurrect a unit (that can only act next round) then for him to pepper you with chain lightning, hypnotize your troops or cast mass curse...
-Ice Shards. Use them first round! The dumb Death Knight spent 5 rounds just getting to me, hacking at the crystals.
-Kill Spiders and Bone Dragons fast. Spiders are a pain at that size, and Dragons are simply easy to kill and they'll be blocking you.
Units I used:
-Royal Snakes (with Feona and Snake ring, even the 50% poison res. of the Undead won't matter...)
-Black Dragons (sneak around the map, in the mountains you're sure to find a place to recruit them, even if you cannot fight the guards, just lure them away).
-Shamen (your only source to heal your dragons, plus use totems + ice shards to block his troops/block bodies. They make surprisinglx)
-Griffin (your meatshields. Go for the Necromancers and Skeleton Archers with them, never mind the losses, they can be resurrected easily).
-Originally I use Evil Beholders as fifth, but they are mostly useless against undead. So, I switched to Black Unicrons. They deal extra 50% damage to undead, are relatively tough and fast, plus resistant to magic. You can get them on the Grey Wasteland from the 'evil' druid after you do his quest.
As someone said above, the key to no losses is to:
1: prevent Karador from casting damage/hypnotize spells
2: leave the crystal and one minor troop intact that you kill every round
3: use Rage Taking+Chargers to get mana back, and resurrect your troops. You need to have Rage Taking on a high level for this, for me it was +45 Rage each time used.
It will take a lot of rounds, and pray that the game doesn't freeze or you don't get a blackout during the battle!
