Yep usually when you get to 4 in some magic school, you start to consider yourself in mid-game, as you can start putting on the battlefield some real magical tactics to support your troops
And triqui about slingers I heard many say that they are useful against some kind of troops, I don't think I can give you a complete answer about what those troops are, but I can tell you as the sling takes only one hand, some slingers have slings in addition to a melee weapon and a shield! If set to fire archers or attack archers, they can shoot them with their slings, suffering much less from the return fire of archers' arrows as they can parry them with the shield, and when they end ammunitions or reach them just start bashing the archers