Respect Roark, for choosing such a difficult endeveaur. The aragonese are quite a handful.

It just might be impossible to win, but I would suggest the following.

If the enemy has 1.000 troops and you have 500, make sure that the machtup is perfect. Say 600 of those 1.000 are spears. Then you train swordsmen. If they're ranged troops, you train horses. You need to balance your troops perfectly to succeed. That heavy cavalry can be taken down in the forrest, or on a good hill.

One tactic to make sure there is a gap between your general and the enemys, is to bring arbalasters, target the general and they'll soon disappear.

You must attack to succed. I would suggest NOT building a navy, as it is a costly long term investment, when you need quick returns.

Defending, especially in rough terrain, is usually much easier than attacking. A swift and bold move on the strategic map will make the enemy withdraw in order to attak you in the next year. Make them come to you. A weakly defended home province will usually not be the target, if you lay siege to a castle. The AI will prioritize lifting that siege instead.

Build only a few archers, 2 or 3, and let them be arbalasters. You need most of your troops to be able to melee when outnumbered.

These are my thoughts, good luck!

/KotR