athanaric made some very good points there. There is a good selection of eastern troops available to you. You just need to find out where your weakness is and compensate for it. If they are using too many light horse archers and you can't fight them then load up your own army with more archers. If they are heavy on the charging cavalry then step up your spearmen. Also keep your cavalry in reserve and then flank them once they have engaged your line. In a direct fight you will be having a more difficult time as their cavalry is superior in most cases.

Also if you can keep your generals alive then losing large battles is good as it will allow you to get the reforms. I am having trouble getting the reforms on my current Seleukid game since none of the other factions seem powerful enough to trigger them for me, even when I deliberatly lose. The addition of heavy Cata horses could be useful for you.