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