MONSTERFURBY'S PROFILE

I'm currently working on a game that depicts battles between armies instead of individuals (similar to the NES series 'Destiny of an Emperor'). I am already using a custom damage formula that is based around relative troop strengths (HP), but my combat system is still missing one thing. When a character (unit) attacks, I would like to inflict damage to them as well so that an attack always carries losses on the attacker's side with it.

What would be the best way to implement this? Any help would be appreciated!