The twist is civilians. Have unarmed civilians crisscrossing the area the whole time. If anyone hits a civilian, the civilian lives and the shooter dies.
You can just tell the civilians to go out and run around, but I see some problems with that. Another way to do it would be to put 5 little orange flags on the field in the pattern of a star and have them run the star pattern right in the middle of the battle.
This would have them cover a large area, moving different directions, and you could set it up so they don't go directly in front of a building where they may run in front of someone shooting from inside who can't see them until it's too late.
The downside is that it would be boring and tiresome for the civilians. You could take people off each team to be the civilians, and once someone on that team gets killed, they could leave their weapons at a designated location and take the place of their civilian teammate.
