You will need 2-4 players online. Play through a campaign on easy and be in an XBox live party chat. At the finale saferoom people who want the achievement leave the game. The remaining player will have to invite the other three immediately, but they wait to join. The remaining player plays through the finale. When the rescue vehicle arrives, throw a pipe bomb or boomer bile so that you can pause it to vote to change the game to expert. At this point the other one-three will rejoin. Fight off the hoard until all the remaining players return.And then complete the finale. The three that left and come back will get it, but the one who played the finale will not.