Iaosatae are quite skilled with their slings. A sling bullet, when slung by a skilled man, will bury itself inside of its target, or break bones.
Iaosatae (Yo-sah-tie; "Slingmen") are a varied lot, some young and some old, who've been educated in the art of sling combat. They're quite skilled with their slings, which are truly a deceptively simple weapon. A sling bullet, when slung by a skilled man, will bury itself inside of its target, or break bones. Either way, the one hit is incapacitated or dead. Impressive for what amounts to a leather strip and a rock. The sling is also inexpensive, and with an impressive range. This allows Iaosatae to be both effective and affordable. However, when the distance between them and the enemy is closed, their low expense is quickly made up for in the ease of their breaking and fleeing, as, armed with only light weapons, they are not intended to fight hand-to-hand.
Historically, the Iaosatae would have been a variety of warriors, mostly young men though with little experience in close combat. Few older men continued to use the sling, it would generally be replaced by a spear or sword, but it never fell out of favor with younger men, who could use it for war, games, and hunting. However, their youthfulness, and lack of experience with the grimmer face of combat, makes them prone to run, because, after all, a man fifty meters away with a sword looks a lot different two meters away. Making matters worse for them, their inexperience in close combat means they probably aren't as hardy as other warriors, with more experience fighting in long engagements. Slings were popular with levy warriors in all of Celtic culture, as it didn't take long to train one in the basics, and skilled slingers who went on to become somewhat elite professionals were often well-versed in both sling and short spear combat, meaning an experienced group of Iaosatae would double as impromptu light infantry.