Instructions
Select a third party member with strong physical attacks. This party member should not be a White Innate Element, such as Karsh or Grobyc.
Equip your party with only healing spells and healing items. You will not need offensive magic, because Dario has strong magical defenses and will counter any spell with a powerful magic attack, killing most of your party.
Attack Dario by selecting any physical attacks from your character's battle menu.
Select "Defend" at the end of every character's turn so that they have a chance to lessen their damage from Dario. In order to do this, you must have a small amount of Stamina left in your Stamina Bar at the end of the turn. Attack only until your stamina bar is almost empty and select "Defend" as your last action.
Study Dario's attack pattern and prepare yourself for his worst attacks. Dario always attacks with the same pattern -- a physical attack combo, the Dash and Gash special attack, another physical attack combo and the special attack Sonic Sword. His physical combo is strong enough to kill Riddel in one hit while Dash and Gash will kill both Riddel and Serge, unless he is in Lynx's body. Sonic Sword is a medium damage attack.
Heal your characters by selecting healing items and spells from the battle menu whenever necessary. Keep your characters healed before powerful attacks like Dash and Gash.
Attack, defend and heal again and again until you have defeated Dario. Patience is important, as Dario has 3500 HP and strong defenses.