My best successes with the Hayasdan involved taking the offense... immediately making a direct, head-on assault against the Seleukids, striking straight into the heart of the empire, taking Seleukia at the end. At the beginning, the Seleukids have their hands really, really full with all of their enemies, and are very, very vulnerable. If you wait to attack, they'll eventually often ally with those enemies (who would rather be attacking you), and then it'll get really bad.