If you equip the Cursed Ring and a set of armor that offers elemental protection. You will now absorb the damage from that element at the cost of reducing your physical defense. For example, when used with the Dragon and Crystal Armor it will absorb all elements.
Avoid physical damage
Cast the Blink spell (White Magic) on your characters to have them immune to physical damage for a short amount of time. Rosa, Kain, Ceodore, and Fusoya can cast Blink.
Completion bonus
Successfully complete the game. Load the cleared game file and continue the game. Go to Namingway to play new challenge dungeons. There is one for both the base game and Rydia's Tale.
Easy battles
Use the Stone Blade, Medusa Arrows, or other weapons that can cause the enemy to slowly petrify.