Just got this as my last cheevo in the game and wanted to pitch in my 2 cents. I highly recommend the method that Sashamorning used, that is having a good team but starting with bad players and subbing in the good players at the start of the game. No need to pull ALL the good players... I left my OL and DL mostly intact but put in bad skill position players. My overall rating was a 69 and every time I was able to beat whoever I was matched with (and I'm not that good).
This achievement really frustrated my at the end because I made it through my season without a single loss and was playing for a chance at the Super Bowl. I think EA matches you in playoffs with other people who are at the same point and YOU HAVE TO REALIZE THAT THEY MAY BE IN A HIGHER LEAGUE (AKA, WAY BETTER)! I went from winning every game that season to getting beat 35-0 and was pissed. Worst part was it bumped me up to the next league, so you're going to be playing better players.
If that happens to you, I recommend intentionally losing every game to get demoted or quitting. I quit most games and didn't get banned, then started from scratch again and won the achievement.
This is by far the most difficult in this game IMO. Bottom line, get a good team, put your good players as your second string, matchmake, substitute the good players, and win. If you lose the game before the SB, just do really bad to get demoted and start again.