I've tried to use armour in the desert and the only chance you have with heavy armour, knights, upgraded feudal and chiv units, is to break the army so badly that their reinforcements give up the ghost really early. Have a good few archers lined up behind your'e by now exhausted main line of knights and spear and you can break the second wave without them getting inside charge range, and usually the AI will give up rather than bring on the third wave.
It doesn't always work though and in that case you'll be slaughtered!
This was in a game when I tried to establish a Frankish realm by abandoning France. Was overwhelmed eventually but there were some brilliant battles in it.