The new roster to use would be hosted by Treil199. It is still for the 49ers. This information is brought to us by user BoTheGr8.
While nasty's Solution is valid and works I have defined the solution to step by step and get these faster then doing offline.
First of all I want to give credit where it is due, this is ported over from the achievement listing for the 360 version (Majority of comments are from SamuelHayne). Also want to give props to Nasty for providing the roster.
Doing it this way should cut down the achievement to about 45 minutes.
1. At the Madden home screen, go to share. Download the Nasty's Roster.
2. After the it is done downloading go back to the home screen, go to the right and go under Customization. Click Manage Files and then load the roster.
3. After loading the roster, go back to the home screen. Once at the home screen go to the Play option and select Connected Franchise.
4. Select Create Online.
5. Name the team anything you want.
6. Under Roster, choose custom.
7. Make your online league private and set a password (doesn't matter what it is)
8. The game will start importing your roster.
9. Once that finishes it will display "Begin a Career" on this option choose coach.
10. Select "Create a Coach"
11.The coach option you want to select is the "Team Builder"
12. In this instance since the modified team is the 49er's you want to select them. If the roster loaded correctly you should see all 4 people rated at 99.
13. At the "Edit Coach Screen", just click "Accept & Continue"
14. At the setting screen the only ones you want to change are the difficulty (set to "All Madden"), and the CPU sign free agents and re-sign players to contract extensions.
15. After those changes go back to the top and select "Start Career"
16. Start online league
17. Start the season
18. Select Advance Week, it will prompt for confirmation.
19. This is what your now going to do for 4 preseason games, 17 regular season games, a playoff, and the Super Bowl.
20. Once the season is completed make sure to simulate the draft.
22. Repeat steps 18-20.
The achievement should take 3 seasons including the Super Bowls. Again full credit goes to Nasty for providing the roster and SamuelHayne for the steps laid out. I just refined everything and put it on the guides for the Xbox one.
If down voting please let me know why so I can learn from my mistakes.