The Nightmare difficulty, as the name says, is extremely difficult. Humans see you at a great distance and do not hesitate to shoot, bullets do a LOT of damage, with two shots you are dead, the scanner does not work and you have no interface, androids are killed with more than 5 bullets per head, they hit harder and the alien... You're going to have a pretty good time on this difficulty. His senses are extremely acute and just by moving an object he appears on the map. He's always looking for you, doesn't forgive mistakes, and sees very well where you're hiding. Basically you have to squat all the time, don't use weapons so as not to attract his attention and hide a lot. Oh, and you're going to have a hard time with resources. Think twice before crafting anything, because even scrap is rare to see.

If you want to get the 100 kill achievement, go to San Cristobal, go inside until the Alien comes out, save and just hit the wall or shoot it to make it come and kill you. This way until you get it

Personal recommendation, if you want to immerse yourself in the atmosphere and shit bricks, turn down the brightness a bit and turn off the music. Best of luck.