The end of the war is at hand and it's time for you to choose which side will survive and which will perish. Talk to Ellie in the Dust and then ride out for the final showdown.
- XP: 4812
- Money: N/A
- Gear: SMG or Shotgun
After talking to Ellie, head out past the door to the Highlands and you'll find a train platform with the two groups staring each other down. When you show up, they'll both tell you to fall in with them and start shooting the other. Pick which side you want to be on, stand with them and then start shooting at the others.
There's no real strategy here except to just keep shooting. Both platforms have railings with metal in front that you can crouch behind for some measure of cover. If your ride is nearby, you can also hop into that and use that to kill them. When one side is dead, the mission will be complete and you can speak to the surviving leader for your reward.