Here is the full list of all 29 Cards of Cthulhu achievements.
Defeat Cthulhu on Nightmare Mode.
Defeat 500 monsters.
Defeat 440 monsters.
Defeat 410 monsters.
Defeat 370 monsters.
Defeat 360 monsters.
Defeat 340 monsters.
Defeat 330 monsters.
Defeat 310 monsters.
Defeat 295 monsters.
Defeat 280 monsters.
Defeat Cthulhu
Defeat 260 monsters.
Defeat 275 monsters.
Defeat 255 monsters.
Defeat 150 monsters.
Defeat 125 monsters.
Defeat 200 monsters.
Defeat 80 monsters.
Defeat 45 monsters.
Defeat 175 monsters.
Defeat 20 monsters.
Defeat 5 monsters.