Use missile units on flanks and the rear of units, or aim for the units with less armour. In my Baktrian campaign they're particularly effective against Pantodopoi and other missile units.

Hetairoi wouldn't break peltasts since they are a spear-armed unit. Use cavalry for the shock effect: keep them in reserve, wear down the enemy and when their morale starts to lower charge them with all the cavalry you can, force a breach. Historically well-formed infantry had little trouble stopping most cavalry, you really need your own infantry to break formations and cavalry to break morale.