Cast Wall on Asura, and she will not be able to use her cure spells on herself, and they will be cast on your characters. Make sure your party is at least level 35 or higher. Attack her with Rydia's best spells and use your best sword with Cecil. Use the Fire Saber because it damages her the most. Note: The Fire Saber is obtained from the Tower of Bable immediately after getting the Earth Crystal.
The spell you want to use with Rydia is Virus. It does at least 700 if your characters are up to level. Also, if you have Kain, make him jump because he usually has the most HP so he does not have to be cured from the reflected cure spells from Ausara. Note: The less people in your group when they get cured , the more HP that they will get back. In other words, Kain needs to be in the air at all times.
If you are one of the gamers stuck in the battle against the Dark Elf, and you are wondering how to defeat it, because he simply causes 9999 damage to each one of your party members killing you without problems, it means you forgot to trigger a cut-scene in Toroia Castle. So, go back there and search for a room where Gilbert in on a bed resting, a scene will trigger. Now go back to the Dark Elf's lair and fight against him, now he won't hit you physically and cause you 9999 of damage, instead he'll use magic against your party, nevertheless you will lose, there is no other possibility, but this time, you won't get a GAME OVER instead Gilbert will help to battle the Dark Elf with your "metallic" weapons. So be sure to re-equip everybody with their metallic equipment and fight the boss.
Select "Item" and press A on an empty slot during a battle. Move up until the equipment in that characters left and right hands appears. Press A on the weapon to be duplicated. Press B and complete the battle or retreat. After the battle, equip that character with their weapon in the same hand it was in. The weapon should have a "2" next to it. Unequip, then re-equip the weapon. You should now have one copy of the weapon equipped and another one in inventory.