There are a couple solutions to the problem of Dark Knights, supposedly the most powerful enemy in the game, being pushovers;
1) Give the dark knights better horses. Cavalry battles in this game are often decided by who has the most manuverable horses. Its kind of funny to see an army of dark knights with hundreds of thousands of denar's in equipment riding around on a bunch of nags.
2) Give them more balanced units. Sea Raiders are good example of this. They are equiped with good armor, good melee weapons, good ranged weapons and are quite possibly the toughest units in the game. The black knights should carry a wider array of weaponry like bows, crossbows, and throwing weapons. Maybe even have a 'Black Cataphract' unit, or just give the 'Dark Hunters' bows.
3) Have the enemy AI reprogrammed so that it 'recognizes' whether an enemy has drawn back his weapon to strike, and what path that weapon will take when swung. One of the easiest AI exploits at the moment is to pull back (but not release) your weapon, then swing as the enemy soldier walks into range. The AI should at least put its shield up.
4) Improve horse physics. There have been dozens of times when an enemy horseman moving perpendicular to me slams into my side. In the game, he come to an instant stop, and it allows me a free swing at him. In reality, either his horse should realize its about to collide with another horse, and stop short, or both our horses go tumbling over spilling both riders onto the ground.
5) Give the AI to ability to pick its battles. If the current one isn't going so well, it should retreat and fight you agian on a different battle field.
6) Make the battle fields more realistic. Most of Calradia is a featureless plain. Where did all these cliffs and mountians come from?