As Pahlava you need no fullstacks, just stick to the Dehbed Asavara (Parthian Noble Medium Cavalry) and half/quarter stacks they are cheap, but fast and deadly.

You don't need the other heavy HA. Try to raid the AS core cities (like Susa, Persepolis, Ekbatana, Babylon, Seleukeia, Antiocheia) you sometimes get 20.000 or even more from one city, that money invest in new Dehbed Asavara (Parthian Noble Medium Cavalry) and some military outposts, where you can recruit fresh units. Take a look in the recruitmentviewer and plan which cities should be you military core cities, you don't need a reformed governmentin every city. If you blitz AS at the beginning, you shouldn't have a problem with it. Ceterum censeo Romam esse delendam.