This achievement is story related and unmissable, just continue playing.
*** Spoiler - click to reveal ***When you've been to Camden to deliver the samples, you'll try to contact the GRE. Instead, you end up talking to Rais, who is daring you to come kill him. To find him, follow the objective to the large tower (you can't do this part in coop and you'll get a warning that you have to finish the final part of the story before you can go back to Harran to finish side missions and challenges you didn't complete so far). After some climbing, you'll find yourself in a part of the tower with a lot of zombies. Just kill the viral so they won't go after you (isn't necessarry I believe) and jump out of this part of the building to your right. From there on, just RUN. Don't fight any infected, you'll be able to escape if you just run. After some more climbing, you finally meet Rais on the top of the tower. A cutscrene will start untill you'll have to fight Rais in a QTE. Press , , , , , and then repeatedly to defeat him and you'll be in your last cutscene of the story mode. Achievement unlockes right away.
Click here to see a video of the last part of the game (credits go to MrRetroKid91):