My tip is - attack the seleucids as early as possible. Make sure to have all your general's bodyguards cavalry, a few pantodapoi phalangitai, and a few archers (eranshar arsthbara or thanvabara) and you should be able to deal with most early opposition the seleucids send after you. After you take the first of their cities you can expect numerous counter-attacks. I advice going south rather than north when continuing with the war vs the seleucids, because you'll want the northern seleucid settlements as a buffer between you and pahlav and yuezhi. You'll need a lot of money and many settlements under your control before you can assemble an army of horse archers capable of wiping out pahlav and yuezhi so it's good to take out as much as possible of the seleucids as quickly as possible. The strategy worked very well for me.