Right.
Do not destroy the crystal at all until you're satisfied with all the resurrecting done. Leaving it alive allows for free ress turns. By using necro call or positioning your own troop above a dead body, you will prevent the crystal from summoning, so it will just skip the turn (provided karador has 0 mana already).
Necro call indeed helps a lot here - even lvl 1 of it. Summon every time you can to prevent the crystal from doing that and to give yourself some scapegoats to take the hits and get free ones in turn without retaliation. After everything's down and you have a bunch of summoned troops already, set the bone dragons on a hex for from your army and start with the poison cloud trick.
If I recall correctly, I ended up this battle at about turn 80 or 90
If would've been more if I had to ress all the priests, but... Whatever
BTW, this trick doesn't fit in every situation. The cloud must be really far away, or it will try to attack an enemy. Try the farthermost hex from the enemy itself and pray it works