Under the advanced option section please add a fatigue trait. In a nut shell under the strength bar of a unit will be a fatigue bar. After each battle this bar will drop x amount. When the bar is empty your unit cannot be deployed to battle. To fill the bar x amount , said unit must not be deployed . This will make you rotate your units some what and not rely on the same units over and over again. It will be up to you if you rest your units regularly or only when necessary