Mod - Improved Light Cavalry

You could give the archery 1.5x hp and a shield defense modifier (Say buckler or Kite without actually having the shield). This would be represented that when an arrow or a sword blow strike the cavalry unit a % of the damage is done to the horse and not the archer.

Without being able to change the straight forward tactics of the horsearchers this is the only way I could think that they would be on par with anything else cost efficent.