Avoid elemental damage
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.
Easy "Twofer" achievement
Defeat the Lunar Dragon in the Lair of the Father B3 in the Subterrane. Next, have Palom, Porom, or Leonora learn Dualcast. Leonora is recommended as she gets Black and White magic and is a better White mage than Porom. Place the character that learned Dualcast in the party and start a battle.