Here is the full list of all 25 The Hardest Dungeon achievements.
Start the adventure for a first time.
Defeat a monster.
Switch to a different hero.
Start the game in softcore mode.
Start the game in hardcore mode.
Defeat Count Vladir.
Defeat General Gore.
Defeat Heart of Mount Kalok.
Destroy the three statues of sin.
Defeat Centurion 360.
Defeat Council Member Ocello.
Defeat Abandoned Surveyor.
Defeat Gatekeeper Titanotem.
Defeat Council Elder Nashrado.
Defeat the Big Three.
Defeat the Triple Turrets.
Defeat the Ancient One.
Defeat the Ultimate Evil.
Defeat the Hidden Boss.
End the game with memory erased.
Challenge the Hidden Boss.
Asks for a prosperous ending.
Failed to defeat the Hidden Boss.
Finish the game on hardcore with Hidden Boss defeated.
Finish the game on softcore with Hidden Boss defeated.