Here is the full list of all 19 Obscure achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Finish the game in 'Hard' mode.
Kill 50 monsters hand-to-hand.
Finish the game in 'Special' mode.
Kill 150 monsters with a firearm.
Finish the game.
Every playable character survived at Leafmore.
Finish the game in 'Normal' mode.
Find all of the weapons.
Cross the gardens.
Kill 3 monsters with a single light grenade.
Enter the gardens.
Check out the dorms.
Search the basement.
Search the auditorium.
Explore the library.
Go to the nurse's office.
Find the principal.
Leave the office building.
Finish the prologue.