All of the achievements are unlocked by completing puzzles in "The Greatest" section. Ctrl-F the achievement 'letter' you need and look at the puzzle title/yellow box in the screenshot to find the puzzle (ignore the cheese puzzle on the left).

If you already beat the puzzle but for some reason didn't get the achievement, THEORETICALLY you should be able to get it by beating it again. I haven't tried this yet but if it doesn't work, let me know and I will do some testing to try and help you.

All the info in this guide is info I got from looking through the game code/files. I already had some of the achievements unlocked so I wasn't able to personally verify that all the info is correct. If you find any errors, please leave a comment.

Puzzle title: 2+2*2

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Puzzle title: Which came first?

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Puzzle title: Enigma

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Puzzle title: Key needed

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Puzzle title: Night creature

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Puzzle title: T.E.

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Puzzle title: Balance

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Puzzle title: Electrocar

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Puzzle title: Pink bird

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Puzzle title: Olaf

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Puzzle title: Without milk

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Puzzle title: Eva

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Puzzle title: Monocle

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Puzzle title: Navigation

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Puzzle title: Smiley face

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Puzzle title: Jumpy logo

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Puzzle title: God of the dead

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Puzzle title: Party hard

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Puzzle title: Who killed him?

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Puzzle title: Maleficient

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Puzzle title: Ben

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Puzzle title: Dark side

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Puzzle title: Flying object

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Puzzle title: Free energy

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Puzzle title: Show me what you gotPuzzle title: Brain dead