Here is the full list of all 22 Mystical Mayhem achievements.
Found Study room key
Found Storage room key
Defeated 1st floor boss
Found Library room key
Found Bathroom key
Defeat 2nd floor boss
Found Bedroom key
Found Observatory key
Defeated 3rd floor boss
Play endless mode once
Reach 100 points in endless mode
Reach 200 points in endless mode
Reach 300 points in endless mode
Play Timed mode once
Reach 100 points in Timed mode
Reach 200 points in Timed mode
Reach 300 points in Timed mode
Defeat 20 ghosts in 1 sword activation in endless mode
Defeat 20 ghosts in 1 gun activation in endless mode
Defeat 20 ghosts with 1 ghost trap activation in endless mode
Unlock Timed Mode
Unlock Endless Mode