Two words: Horse archers. Get lots of them and get some round the sides and back. They do lots more damage shooting the unshielded sides of phalanxes. Then when you are out of ammo (it helps if you have already destroyed their cavalry or lured it away from the fight) charge in. The morale tends to be lowered, and they rout quite easily. If they dont, withraw and then charge again. As for killing generals, I just overwhelm them with arrows. I try to avoid fighting them in a fair fight, as they will win! The key to Hayasdan has to be their mobility and range. Treat the cataphract archers as normal horse archers, with the bonus that they can charge and do some serious damage (ie when you are ready to rout the enemy) and you should prevail...
Wait till the Seleukids start throwing Kataphracts at you - they are so well armoured that arrows have very little effect. It makes taking down generals look easy!