KingCobra1064:
Instead of thinking in terms of 'Harder,' perhaps to think in terms of more interesting.
You have struck something valuable with your comment; "I also found that it is too tedious to destroy all of the castles."
By just making large fixed forces and positions is not making the game harder, it is making it more tedious.
Yes, there is a lot of fun to be had with a large fixed force battle, but I suggest you start to think up of interesting and balanced play on a smaller scale (much less units and buildings).
Use the units of your choice and create a microcosm of the larger battle in your prototype. In other words to find out the nature of the units, and buildings (their strengths and weaknesses).
Set the enemy position as the strongest (research, age, etc.), and find some creative ways to win against the stronger force if possible, or add creatively some strength to the Player One position until you can 'just' defeat the enemy position. Take note of how you achieve victory to instruct and hint at in your game. Once you have a challenging battle on a small scale, you can then create your large battle from it (by multiplying the aspects for each position).
There is much to know, and I am concerned that what I just outlined above will be useless to your level of orientation with the AoK editor. Therefore my best advice would be to play more custom scenarios/campaigns, and to find articles and utilities that teach general aspects of scenario design.
"I take it that this is the Anastasia Scud pines for?" - Epic Commander
"What Ana said. Use sugar and the whip." - aka the Pilot
"I think you will realize the emphasis was on Ana and Cake." - Monk