Here is the full list of all 20 Days Like A Nightmare achievements.
Clear DAY1
Clear DAY2
Clear DAY3
Clear DAY4
Clear DAY5
Clear DAY6
Clear DAY1 HARD
Clear DAY2 HARD
Clear DAY3 HARD
Clear DAY4 HARD
Clear DAY5 HARD
Clear DAY6 HARD
Clear DAY1 NIGHTMARE
Clear DAY2 NIGHTMARE
Clear DAY3 NIGHTMARE
Clear DAY4 NIGHTMARE
Clear DAY5 NIGHTMARE
Clear DAY6 NIGHTMARE
Shoot Something You Shouldn't
See the Secret Ending