Either add to it the item ability - Item attack fire bonus or edit these fields in the Combat of the unit:
Combat - Attack 1 - Area of Effect damage (Full damage)
Combat - Attack 1 - Area of Effect damage (Medium damage)
Combat - Attack 1 - Area of Effect damage (Small damage)
Combat - Attack 1 - Maximum number of Targets
Combat - Attack 1 - Area of Effect Targets
Combat - Attack 1 - Weapon type : "Missile (Splash)"