A lot of cavalry apparently. A core of pike-armed phalanx, some thureophoroi on the wings, well armored. Possibly a unit of pheraspidai or agema. Sometimes elephants, sometimes not. But the best part was probably cav. Seleukid-like hetairoi along with the cataphracts and some light cav.

The problem is we don't have a lot of good details from multiple battles. One big one was almost all cav, at the Arius river. Good Baktrian cav and hetairoi in formation attacked the seleukids but couldn't win the day. It's been implied that the infantry was not brought along for fear they (mostly greek) would defect. But the more native baktrian cav units and hetairoi were less likely to do so.

I think one more baktrian cav unit also is in current production right now by the way.