Hi, As I think you realize, a writer commonly expects his reader to use common sense about meaning. If the writer intends some strange and unusual meaning, rather than the very obvious one, he will usually reword his sentence to make that unusual meaning clearer. Now that I've said that, let me add that, to me, this use of the phrase 'difficult achievement' is unlikely to occur in modern English.
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AnonymousIt might seem as if he is saying that concentration is essential to make achievement difficultNo. The correct paraphrase is "... essential for achieving difficult tasks".