Even though Nightmare was introduced in a post-release patch, you only need to complete the game on the Hard difficultly to unlock this achievement.
With Hard, you will have fewer ammo and item pickups (although this isn't really an issue as long as you're doing reasonable exploration and checking corpses), the enemies will detect you easier, will hunt for you more aggressively and you'll take more damage from bullets and Joes. The Alien will also be more aggressive and will stay closer to you as you navigate levels, and come Mission 17 the Alien becomes incredibly aggressive.
stacks against this achievement.
Unless you feel like being truly challenged, stick with Hard as the difficulty for your playthrough. But if you want to try on Nightmare, here are some of the changes from Hard:Alien: Isolation now comes with a new "Nightmare" difficulty mode that includes a deadlier xenomorph, harder hitting androids and more aggressive human survivors. Players also have to contend with fewer resources and can no longer see their health bar, ammo count or flashlight indicator. To make matters worse, players have to use a broken motion tracker that gives them "unreliable information."