My last great achievement would be told in a linear narrative where I would explain how it happened and how important it was in my life. I would also talk about the difficulties and everything that prevented me from achieving this achievement and the ways I found to overcome these difficulties and achieve this great achievement.
I would tell this story through an email, because it lets you send the story more easily, as well as allowing reading to be done more easily. In addition, emails can be done less formally than a letter.