Armed with javelins and an axe, and armored only by a small buckler, these soldiers, like most Eastern infantry can not hope to survive in a fight against either settled infantry or nomadic cavalry.
Many Iranian peoples inhabit the hills and mountains around Central Asia. As the nomads advance into these lands, they often avail themselves to these tough hillmen who can operate in lands that even a steppe pony may have difficulty. Armed with javelins and an axe, and armored only by a small buckler, these soldiers, like most Eastern infantry can not hope to survive in a fight against either settled infantry or nomadic cavalry. Even so, they can do heavy damage against unwary opponents, and are far more manuevarable than armor-laden heavy infantry. Willing to fight for most people who control their lands in return for money and booty, they can help the often infantry-challenged nomads even the odds in more conventional battles as well as sieges.
Historically, Baktrian hillmen or at least soldiers like them have been around in the same lands for many years, serving the Persians and now the Greeks. From the earliest states to emerge in Central Asia to the recent establishments of Hellenic kingdoms, troops like these have often been levied for support infantry duty or just to fill the ranks of the army with greater numbers.