Which is stronger, Death Harbingers/Banshees or Death Knights?

Peace! Humans are simply evil. Although I can still handle them and their training, elves are a different story... Their initiative and hidden archers are terrifying. I recently fought on a huge map. I had 250 dragons, and my opponent had 1900 hidden archers. The archers attacked first, and my dragons were gone, plus his hero cast Haste on his entire army! He killed my entire army with those archers!
And what about the hero development algorithm? I've already tried all the development combinations, but none of them guarantee a 100% victory. Usually, I study Attack, Defense, Education, Black Magic, and Summoning Magic.