If you have roughly equivalent forces, you can beat larger opponents by luring them to the coast, preferably in a mountainous or river crossing situation, and having huge backup armies waiting in boats in the sea. When the opponent's army is 4-5 days away from landing, send your ship-based units out to the province. He'll think he has an easy-win situation, but in reality he'll be ambushed.
Here you go,
Weakening their numerically superior forces:
Defeating them when roughly numerically equal:
For my victory, I had mountainous terrain + river crossing + a stacked central column + godlike generals (sort through the character list, by martial ability, for unlanded males of your religion. By the time the Hordes arrive, there should be some good ones willing to come to your court, in the hope that you'll press their claims).
I forget whether I stacked my central column for the weakening attack. Probably not, but probably should have. Saving before a big clash and experimenting with different army configurations is a good practice, but one which I rarely bother with anyways. I did it to get my victory though, because I saw that my central column collapsed during my first try, and decided to see what would happen if I bolstered it, to good results.