If you haven't unlocked the achievement by playing through the game naturally, there is a method to fix the broken DLC. Head to your dashboard and delete the DLC from your HDD. When it is deleted, head back into the game and reinstall the DLC. If you replay the game from the last checkpoint and finish, the achievement should unlock without any problems.
This achievement does stack with Survivalist, so you can knock out both Ranger mode achievements by starting a play through on Ranger Mode Hardcore.