Go to Johnny Appleseed Shiny. Then, go to the next save game location, and fight books and monsters to get from level 15 to 25. Even those you might die many times, you can continue on the battlefield without going back to the title screen.
In the town of Laila Bell, there is a black market where you can trade some of your characters' levels in exchange for items, such as armor, weapons, level apples, attack up apples, etc. While this may seem useless (a level apple costs 10 levels to buy, a level 20 apple costs 100 levels to buy), there is actually a very easy way to work up all of your characters to level 100, as well as get as many attribute up apples as desired and any equipment the black market has available. For whichever of the three characters you do not usually use as a part of your party, sell off all of their levels. You can only take them down to level 1. Use those levels to buy as many level apples as possible. If you can afford a level 20 apple, do that instead. Once you have bought as many as you can, leave and go to an area with monsters as tough as you can take on. With the Growth Egg Badge and the Hen Badge on your main three characters equipped, complete several battles. Your three other characters will get the rewards and gain more levels. You can use Lucky Cards during all of this. Since your useless characters are starting from level 1 each time, they will gain their levels very quickly, which will save you a lot of time in working up everyone else. Return to Laila Bell, and sell all the levels again from the three characters you do not use. Then, use those levels to buy whatever desired. If you buy level apples, you can work your main characters up to level 100 very quickly, then take them to strong areas such as Abyss, Inferno or the TF System Towers (depending on your strength). If you set up your badges correctly, you can earn up to 40 levels or higher per battle for your useless characters, which is 120 levels per battle that you can sell towards apples or whatever you are buying (without the use of a Lucky Card, which should double the experience and speed up the process). When you are done leveling up your main characters and buying whatever equipment or badges you need, save fifteen level 20 apples, and bring your three other characters up to level 100. You should have no problem taking on any monsters in the game, and if you spent some of your levels buying attribute up apples, you can make your characters nearly invincible. You can also use this method to buy some of the badges from the black market, and along with synthesizing, you can easily make as many Sheriff Stars as desired, or any other rare badges you need.