While you can complete the story at Level 55, there is still a lot of endgame content that goes well above Level 90, such as the Tainted Monsters #51-60 and 10th Dreamer's Door Maze (which has a Level 95 boss). After getting a 99% game completion (all tainted monsters, side quests, and story chapters), you will just be around Level 75. That is still a long way from standing a chance against the toughest monsters. The quickest leveling method is to repeat the final story boss. He can be replayed an unlimited number of times and gives half a level-up each try (takes approximately five minutes). To maximize the XP earned, set the Tactics Tweaker to favor XP, craft 3x "Necklace Of Enlightenment" at the Evermore Outfitter, and equip it to your active characters.

Also, research everything at the Basic Training Ground and Officer's Training Ground. Finally, assign six people to the Sage's Salon. During the boss fight, skip through all cutscenes, lock on to the boss, and hit him with fast attacks. You can also do other things while doing this, such as watch TV. Just press one button for a few minutes. Skip the cutscenes after the boss, save game, and reload from main menu. Repeat this as many times as desired. It takes approximately six hours to get from Level 75 to 95 using this method. At Level 95, the 10th Maze is quite easy. You can probably do it at Level 85-90, but since you must go through 30 floors again if you die, it is recommended to do it at Level 95.