I've seen heroes die form what looked like damage. However, the reinforcements might have fallen below a certain level. I know one time on Bespin: Platforms I was the only troop left on the field for my team, and the other team had about six or seven. That usually wouldn't be too hard to win, except Count Dooku was waiting for me in one of the crucial choke points. I don't know how much lower the reinforcements had to get.
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