Actually I disagree with that last bit of advice: Defensive battles are astonishingly easy in Medievil, you can Just sit tight somewhere on a hill or where the terain gives a natural advantage: in a valley say or next to some trees.

This way, you can take bucket loads of missile troops, (usually about 2/3s of your army and some very good defenives ones and let the enemy do all the work. By walking all the way across the battle field towards you they will tire more easily and tired troops route quicker. Let you archers do all the damage as bunched up troops are easier to hit and when they do actually get close enough target individually, have a very good archery unit (Arablests or Longbows) target the general specifically. This way one will break and run and both armies will actually get in each others way.

That is one way of doing it anyway. However, yes it is tricky, but then again: that is why we thought that generals thoughout history that could beat opponents twice their size were good....