The last elemental has to be defeated quickly, or you will not survive. She is weak vs holy. Unfortunately, all other elementals are strong against holy, so using the Light Sword (meh as a weapon) or the Excalibur (if you didn't do the optional quest, then you're missing out a bit) should be painful until you reach the last elemental. However, you can work around this by casting Berserk on Cecil. Alternatively, you could just equip the Avenger Sword and Cecil should be effective throughout the battle (it doesn't have Holy elemental, but auto Berserk makes Cecil very dangerous overall).