I think that the problem isn't that slingers have crushing power ; the problem is that this isn't taken into account in the balance. For the moment, slingers stats are somehow in between the archer and the javelinist (on speed, fire rate, attack and range) . But the fact they can destroy isolated buildings (towers mostly) gives them a strategic power which isn't considered in stats : Whereas the other units cannot stay under the enemy towers too long, the slingers can just destroy them (or almost ,then capture).
Also, i was wondering if slingers are not benefiting from the best balance between range and fire-power : whereas long range (archers) can induces (favorites) the phenomena that the fire-power is wasted on only few closest enemy units (arrows in an already dead unit) ; the short range of skirmishers make them bumping in each other and even more when encountering obstacles . In other words, slingers are, maybe, for now, on an optimum and not simply a linear average between archers and skirmishers.
For next version, rather than making slingers stats simply between archers and skirmishers, on the 3 stats, I advice to :
- Move speed : swap skirmishers and slingers (which make sense as the slingers are very light armour)
- Fire-power : swap archers and slingers (which make sense, an arrow is blatantly more deadly than a stone). (But maybe also consider the strategic advantage of crushing power.)
- Range : keep it like it is