Rising Champion Achievement

Hard difficulty is unlocked after beating the story. After loading your post-game save data, you can return to your base in town and change the difficulty to Hard (and back). None of these bosses are really any harder, especially if you have maxed out your weapons/armor, completed all the quests to get the infinite links in battle, and equipped items to guarantee a critical hit on every character. You can add in potions for extra attack or defense, or get buffs from the Inn, but none of that is really necessary. All of the bosses respawn each time you leave a dungeon, so you can easily farm them for items or experience.