In order to make combat more realistic, is it an idea to give the saboteur unit line, or a separate unit, the ability to leave a battle mid combat. This would enable them to enact true hit and run guerillia tactics. This would make them very strong units yes. But then again the guerillia tactics have beaten many large invasion forces and long term oppressive regimes. I would even concider to give the ability to heal in territory that is occupied. Maybe some limit is in order to balance. 

I love the idea of guerilla tactics being used in a mid-battle setting. It would definitely be difficult to balance!