Expert (45.8%)

Clear any level on expert difficulty without any modifiers.

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Guide written by a human:

To unlock this achievement, all you need to do is complete any song on expert difficulty without using any modifiers, just as the achievement description states. After you get comfortable enough with hard difficulty, this one should not require much more effort. After you finish the song, the achievement will pop.

Once again, this can be done with your own custom map, though I don’t think it’s needed.