All credit goes to BladeMDP from x360a.org for sharing this method.
First thing, head over to SHARE and download my Roster. Search for my GamerTag (Neditch), and download Roster File "NINERS99". This Roster has every player on the San Francisco 49ers maxed out to 99. This took nearly 2-hours to do, so your welcome!
Next, start a CONNECTED FRANCHISE as a Head Coach, making sure you "import your roster" prior to creating your coach. Remember to create a new coach. It probably doesnt matter, but choose the option that has Joe Gibbs on it.
Now, once you get to settings, do not change anything other than the difficulty to "All Madden," and allowing the CPU to sign free agents and re-sign players to contract extensions.
Now, do not play the games. Simply advance each week. If you win, save and move on. If not, rinse and repeat. (I did not lose a single game in 3-seasons). It will take three seasons at minimum to earn all the legacy achievements. Just make sure to go undefeated and win the Super Bowl each season.