Have you ever noticed how the first one or two volleys from slingers or archers seem to not do very much, yet each volley following that seems to get more and more devastating?
Yes ive noticed since the first battle i gave in the first campaign, where as macedon fought the thracian faction just to the east of my borders.
I had 4 units of slingers in the army, i remind you also the problem with the units speed on the first days after the release, so after two rounds, my slingers started the retreat, since the enemy was already upon them??? with only 1 dead among them..

They definately must do something about the casualties inflicted by slings, ecpecialy among armored units.