Just before you do stay on your feet sparring, go to options and game options first. Set the power bar to empty and the toughness bar to max. You will now be able to do the sparring session without getting knocked down. When the sparring session ends, go to options again to change the power and toughness bars back to normal.
To do this you must first ensure that autosave is off. Then, exit Legacy mode saving. When you load your legacy again your popularity will have been restored and you have not and will not be challenged again.
Start a new legacy and win the tourney going to 'Punch Totals' in the last fight and pressing circle. This will give you +5 added to your power, stamina and heart. Keep repeating this until you no longer get stat upgrades.