Simply put, you get this achievement after finishing the campaign on ANY difficulty. It will take between 5-10 hours depending on your level of skill and difficulty (veteran will take slightly longer I imagine).
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Just as a heads up, after the first set of credits there is a little cutscene related to the story, and then after the second set of credits, there is an easter egg - so don't go to the toilet or get some food whilst waiting for the credits to finish rolling!