When you reach Fletz and the Twin Towers, you must go on a quest that ultimately ends you back at the castle with the two princesses right before the Moon Dagger is passed on to the fake princes. Before you enter the Chamber of the Sun make sure you have saved your game and have plenty of Healing Breezes, Healing Potions, and Angel's Prayers in stock.

Make sure you have people in your party with the highest max HP. Keep your HP as high as you can without going through turns oh Healing Breezes and Potions. You might want to consider having Rose in your party because she has the Astral Drain magic which can heal your entire party and do damage to Lenus at the same time. Keep hitting with everyone until you can get a special. Your best bet is to use the special of the person wit the strongest attack. That way you can use your Dragoon Attack and you will automatically pull of the finisher (a perfect.) When Lenus moves her hands and draws a Chinese letter in the air hold on to your hats because it will do serious, I mean SERIOUS damage.

If one of your guys will be knocked out, and then with only one guy standing and really weekend you can either heal him, then get hit again, or use an Angel's Prayer and have both of them knocked out.