Thank you to the great people who provided useful info on this tough achievement. I was lucky in that I hadn't played any online and XBL has free Silver access to multi-player Apr 1-5 so no trial cards needed.
Here's a rough outline of what I did based on the information provided by others in this thread:
Created 5 Silver accounts using my PC (probably faster than on the XBOX).
Recovered the first two, S1 and S2, on separate XBOXes (I have two copies of the game).
For the first boost, I played S2 vs. S1 for 32 matches and got to level 24.
Then I recovered profile S3 and played S3 vs. S2 for 30 matches and got to level 36 with it.
Then I recovered profile S4 and played S4 vs. S1 for 5 matches to get to level 13. Then I played S4 vs. S3 for 21 matches and got to level 43.
Then I recovered profile S5 and played S5 vs. S2 for five matches and got to level 25. Then I played S5 vs. S4 for 20 matches and got to level 47.
Next, I played my first gold account as follows:
- G1 vs. S1 for 5 matches for level 12
- G1 vs. S3 for 5 matches for level 33
- G1 vs. S5 for 24 matches for level 50
Finally, I played my second gold account as follows:
- G2 vs. S2 for 10 matches for level 28
- G2 vs. S4 for 10 matches for level 41
- G2 vs. G1 for 14 matches for level 50
Result: Two G.O.A.T.s got
As others have pointed out, you do not need to play a single point, just forfeit the match for the profile you want to lose.