Well my experience with Hayasdan on H/H difficulty with BI.exe was somewhat different. Playing as them was really a great fun - now I am around year 200 BC and still enjoy it very much - especially thanks to the reforms feature. The difficulty though was not that extreme as campaign description suggests. As Hayasdan you have quite a lot rebel cities around where you can expand at the beginning of the game without starting a war with other faction and setting your economy on a sound footing. On the other hand Pahlava has no such obvious opportunity for expansion and economical growth. Moreover, Armenian basic units are extremely cost effective and combination of Kavkaza Sparabara + Caucasian Archers + general bodyguard can deal with anything your enemies could throw on you early on.
IF you manage to capture them BEFORE Arche attacks you, you are basically over the hill. Taking Karkathiokerta (Sophene) completes the requirements for Caucasus Kingdom reforms - very nice gameplaywise - and you should have a decent economy by then. Therefore I would recommend a blitz through Caucasian rebel provinces at the very start (except Karkathiokerta), than wait for Arche´s treacherous attack, punish them by taking Karkathiokerta and after that a measured - "historical" - push against Mesopotamia and Persia after is taken... Hard campaign difficulty is probably advisable and can be somewhat compensated by hard battle difficulty as well.
In my campaign Archie even accepted cease-fire after I conquered all Mesopotamia (Edessa, Arbela, Seleukeia, Babylon, Charax, Ekbatana, Perseopolis, Apameia, Gabai) and we live in peace since that time for more than 30 years or 120 turns, which is amazing for RTW and I guess that the hard campaign difficulty AND bi.exe are doing the trick!!!