Here is the full list of all 6 Diamond Deeps achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Complete level 21, the Lode Runner man.
Play through 5 levels of a custom mod.
Build a mod with 5 valid levels.
Complete a level with 3 ghosts in it.
Be struck by a Fireball while encased in ice.
Complete a level with Ghosts on it without trapping any Ghosts