Sometime, and if you check the web you'll find many cases, you can experiment a bug in the Fix'er Upper mission. You can go trought the storyline without getting the mission (and so the achievement) and you should find the mission freezed.
If you're in this case, the only way to get the achievement is you create a new character with a second controller and a second xbox profile; now start the game in split screen (be sure to start with the newbye) and join the game with your usual character.
Follow the mission step by step; you can use the main character to kill quickly the ennemies, but be sure to use the new one to speak with Zed. When you end the mission, you'll get the achievement with the main character.
Of course you can ask help to a friend, but I think it's better to do that alone. It works for sure (just done yesterday).