Challenge Dungeons:
To unlock new Challenge Dungeons, beat the game and then Namingway to play them.
Blink spell:
Cast the White Magic spell Blink on your characters and they will become invulnerable to physical attacks for a certain amount of time. Rosa, Kain, Ceodore, and Fusoya have Blink.
Paralysis:
Plenty of monsters are very hard to defeat, but if you use weapons that can cause the enemy to slowly petrify, they are more manageable. These weapons include the Stone blade and Medusa arrows.
Absorb Elemental Attacks:
Equip a Cursed Ring along with a piece of armor that protects against a particular element (i.e., Diamond Armor protects against Thunder/Lightning), and you will absorb the damage anytime that element is used against you. This is especially useful in the endgame, when you get armor such as the dragon and crystal armor, which defend against all elements. Be careful though. It greatly reduces your physical defense.