So, recently I found out that the most effective weapon against Death Knights is a dozen fairies. Basically, we're defending the castle, we have about 30 elven archers, some siege equipment, three towers, and a dozen fairies. The enemy still has about ten Death Knights and 200 zombies. Anticipating the imminent end, we send the fairies out as kamikaze units and have them attack the Death Knights. So, ten fairies managed to kill them in about ten turns, as you can imagine. I didn't expect such an effect from the fairies, but we also managed to shoot down the zombies and Death Knights with everything we had, and we still had some fairies left :)