15 3 0*Spoiler Alert*Story related and cannot be missed.*** Spoiler - click to reveal ***After you get on the bridge there will be an section where you and Luke get ambushed by walkers. After you defeat them you will move on and find a person at the other end of the bridge. After a talk with the guy, Nick will show up and shoot the man. When done talking with Nick a cutscene will begin and the achievement will unlock.