For this achievement, I downloaded the "Dallas99" roster from the community section and edited the ratings of the starters on the San Diego Chargers so they were considerably bad. That took about 3 hours for me to complete, but it was certainly worth it.
Once your rosters are all set, load up the skills trainer until the Chargers are listed as the away team. That took quite a few tries since it appears to randomly generate your opponent. I played as the Cowboys with their players rated at 99.
After trying about 20 times with normal rosters, I was able to get this after just 2 tries with modified rosters. If at first you don't succeed, try and try again. You'll eventually get it.