How should I develop my units to defeat a Knight?

First of all, I'm talking about the late game (around the 3rd or 4th month). He might be weaker earlier on, as he's a late-game hero, but a skilled hero playing as Dugald won't easily give in to you and your Kaspar... if you rush in too early, you'll get hit by rapid-fire ballistas, and the Knight's attack and knowledge stats increase very well... just like Vittorio and Laslo will destroy you with their lancers or zealots... You're talking as if you only develop your game for 1-2 months, building up your army, leveling up, and then clearing out the defenders at the entrance to the Dungeon or the upper level, and then attacking (which makes sense, knowing that the opponent doesn't have enough Knights yet)... play for longer and you'll find that it's also beneficial for Axel to sit and train Knights!
Secondly, you fell for the trap of not knowing the artifact that protects against blinding (SHAME!)... if I remember correctly, the King of the Dwarves' helmet protects against blinding! Play more of Heroes of Might and Magic V: Lords of the Horde....
Thirdly, there might not be artifacts against mind control... the Adepts' enchantment removal will be easily countered.:)
Fourthly, cast your vampirism on whoever you want when you have a good advantage (a really big one); the Knights will destroy your two stacks, or 2/3 of your stacks from two stacks. If you mean spirits, there are also soldiers against them; at least the Knights will simply tear through your army, so you'll be afraid to attack them, knowing that you'll die from a counterattack (exceptions: vampires or spirits)!
If the game isn't in the late game, it's better not to choose Axel:)