One strategy I used was to abandon my Persian satraps at the start of the game, this means the Parthians/Bactrians take more time defeating larger Eleutheroi stacks whilst having the benefits of allowing you to disband alot of garrisons you no longer need (freeing up revenue for a succesful war against the Ptolemies) and shriking your eastern border so it is easier to defend (like the Hindenburg line in WWI). When you have secured Egypt and if possible Anatolia you will have the revenues to afford a powerful Hellenistic army which can steamroll over the Parthians.