Easiest way for me to get this achievement was to select a Prototype CB and play for the Dallas Cowboys. (You start as an 84 or 85 overall). I left all the settings default except the difficulty which i changed to ALL-MADDEN. When you start the league turn the off-sides slider down to 0. When you begin the season just keep playing games till you reach around 40 sacks. Once done save and sim to the end of the season. Just make sure you win the MVP award as that is the one that will make your legacy score jump at the end of the year(If you dont win the MVP just back out and re-sim). I averaged about 2600-3000 Legacy score a season with this method. Hope this helps