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Download the user manual here.
From the middle of the third generation on, if you proceed without careful thought there's a risk of getting a Game Over. Even in Easy mode.
Enemies will start using powerful attack moves during boss battles, so you will no longer be able to win with power alone.
The best way to proceed is to reinforce your equipment, make the most out of the Enchanted Field you're using and the extended areas of the characters in play, and use effective skill sets.
The ideal level for your characters for the second half of the third generation is 45 and up.
If you feel that still isn't enough to fight bosses, "class up" your characters. By classing up at level 50, your AP, slots, and status increase.
You'll also obtain rank-3 Extra Skills, which are extremely powerful and very useful during boss battles.
It's an easy enhancement, so it's recommended to class up your characters once they reach level 50.
An item called Proof of Valor is needed for classing up. You can obtain one after gaining a title.
Note: Proofs of Valor aren't always awarded when you obtain a title as bonus items vary depending on the title.