Originally Posted by MarcusAureliusAntoninus
Go on the defensive. Conquer up to a small border (Syria, Sophene, and some border of Armenia is quite easy to defend from the east factions), then build armies with lot of archers and a few phalangitai. When they seige your border towns just wait for them to attack. When they do, set all of your archers to fire-arrow. They should destroy most if not all of the enemy seige equiptment. Move your phalangitai to defend anything that gets through. Most likely they will start sending crap after a short while (Pantodapoi and such), which is vulnerable to missile attack.
This tactic will mean you only have to fight a battle about once a year (though a seige battle), and you can meanwhile build up a large force to counter them, which I suggest you do in spurts.