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This achievement can easily be gained by searching for contracts in the Contract menu. Set the target number to 1, the level to "The Final Test", and search. Just check the Leaderboards at the plan menu of any given contract (where you choose weapons/tools, up top, next to "Challenges"). If it has no entries or a low one you could beat easily, simply complete that contract to an adequate degree. You may find contracts marked as for the achievement, but if none are available or they consist of high scores, try another random contract. Shouldn't take long.