I just started a Hayastan campaign today, and heres a nice tip for you.

Get Scythian HA as soon as possible! My core army as of 259bc consists of 3FM's and 4-6 units of Scythian HA. I've destroyed anything AS sends at me. I am about to take Persis, Babylon and Selucia itself with just this core army. Of course if you like challenges, only use 1 FM.

Also with Armenia, at the beggining of the game, send a diplomat north and create an alliance with Sauoromate, that way you wont have to worry about HA's constantly harassing your northern borders.