One of the new things in this campaign is that the finale requires one of the players to "sacrifice" themselves in order to complete it. First, you have to activate three generators scattered about the map. To do so, approach one and hold . After the bridge is lowered, you will have to press a button on the platform. Doing this will cause a generator to shut off. One player will have to leave the bridge and re-activate the generator while the others must stay on the bridge.
To obtain the achievement, the person playing as Bill must go down and re-activate the generator. This can be done by yourself easily by selecting "Port Finale" as Bill on Easy difficulty on Single Player. Get up to the finale and proceed to activate the generator. If you are on SP, you must do this, as the bots aren't programmed to do so.