I mostly try not to fight them. Putting archers (and if I'm really pressed even slingers) on stone walls should do the trick. Other than that: maybe consider using phalangites where you only need them (like one unit of pantodapoi in each frequently besieged city). Parthian spearmen and even massed pantodapoi should hold your line. If they don't, so be it. You'll lose several cities from the start onwards, and armies like the one you're experiencing might cost you another, but once you have a solid line of stone-walled-cities, you should be capable to hold your ground. Launch counteroffensives wherever Parthia seems weaker.