Whenever I've played Baktria or Haysadan my armies always seemed to be half filled with slinger and archers of all stripes. Properly used you can shred even elite phalangites with them, and a dense screen of slingers is something even cataphracts will feel (the archers are conversely very good at doing away with the assorted lighter chaff, leaving the sling-boys free to do their can-opening job).