In the main menu of Legacy Mode, go to the Legacy Goals tab. There is a checklist that will show you exactly what you need to do to retire as the GOAT. You will need to retire when all of the following are checked off:
- 90% winning percentage or greater.
- 95% popularity or greater.
- Over 50 fights completed.
- P4P (pound for pound) rank exceeds 5.
- Defend titles 8 times.
- Hold at least 3 belts.
- Win at least 2 belts in a higher weight class.
All of these will come through natural progression of Legacy Mode. Make sure you attend events and win. If you are playing on easy, this won't be difficult at all. Don't forget, you need to retire as the GOAT, so don't retire early.
After your 51st fight if you have completed all the tasks above, retire your fighter and the achievement will unlock.