With Baktria I found the most effective army composition is archer-spearmen (a lot of them), phalanx (Panto-Phal is enough), a FM or two and some medium cavalry.

Horse-archer armies don't stand a chance against this force because they are outclassed in firepower by the foot archers and can't charge against the phalanx either. Even Seleukide armies have serious troubles against it because the archers will kill all their support units before the phalanxes clash, leaving the field to your cavalry.