If you do all the optional quests by the end of the game you will be around level 28-30 and have the True Hero achievement already. The most efficient way to ensure you do not have to grind much before getting Legendary Hero and finishing the story is to obtain one of the treasures. Once you get to Africa, make sure you dig up the Harvester in the second northeast cave. This is a weapon that boosts the XP you get per kill. Use the Harvester from then on whenever possible. In case you did not get to level 32 before removing the curse from the door before the last fight, go to Africa and kill Baka Beasts or alternatively keep entering the arena (talk to Gero in the Jama village) and kill the contestants there. You have to reach level 32 before attempting to kill Victor to get the achievement.