This can only be achieved in New Game Plus.
Unlocking this achievement is slightly different from upgrading Swords or Shields, as to upgrade an accessory, you must 'acquire' it again. The quickest way to do this is straight away in Luxerion North Station. Simply climb the ladder with the man at the top and jump to the next part and take the treasure sphere. Assuming you collected it last playthrough, you should now have a Fighter's Emblem+ and this achievement.