Edit: If you want the achievement super fast, use RG BzN's guide. However, if you want to actually play the mode, here's how the achievement works.
In Road to Glory mode, go to Stats & Records>Leaderboards>Offline Leaderboards>NCAA. Here you will see the Legend Rank score that you need to beat for your position.
As a QB, I needed to beat a Legend Rank score of 2100. My achievement unlocked at the end of the ACC Conference Championship game in season 2 when my Legend Rank score reached 2135.
If you play as QB, the best way to attain the achievement is to always audible to a pass play. Throw the ball deep as much as possible. Score as many TD's and get as many wins as you can. End of the year awards and bowl wins help increase your score also. You should easily hit the mark necessary by the end of the second season, if not early in the third season.