Karador is probably the hardest fight in the game.
First of all, you need to level up. I think I was close to level 27 and had cleared pretty much everything before I took him on.
Now, in the Russian version 1.5.0 when I last played it (where the mage class was
substantially weaker than in newer English versions), I used
Red Dragons (RD)
Green Dragons (GD)
Cyclops (CY)
Elven Pathfinders (PF) (dunno if that's what they're called in English version, you know, those stronger archers)
Archmages (AM)
Using Tactics I positioned my troops at the corner of the starting grid as such
Code:
<> ------that's the crystal
| |
___| RD GD|___
| CY PF AM|
--------------------
Basically, hardcore turtling/weakening of units until the crystal is killed. Crucially, the Red Dragons were at the center and inline with the crystal, so that they could snipe the crystal with their fire stream. Archmage shielded the dragons that took the most damage, Pathfinders shot the crystal. All units are level 4-5, so they won't be hypnotized. Once the crystal is dead, the enemy's mana won't regenerate, so it finally becomes possible to finish his units. Low power Blind, and Target (on your archmage-shielded unit) are useful for keeping the damage of skeleton archers under control.
Holy Light - again, I think that's what the name is in English version - spell is doubly good here: focused on the dragons its AE effect both rejuvenates the friendly "tanks" and damages the attackers.