Instructions
Place Dark Magician in your party. You can win two Dark Magicians over the course of the game. The first comes from defeating Panik, the boss of "Beasty Badlands," which is the sixth mission in the game. You can win a second Dark Magician by beating Arkana at the end of the sixteenth mission, "Lost Cause."
Place Dark Magician Girl in your party. You must have Dark Magician in your party to win Dark Magician Girl. Liberate both of the captured towns during the tenth mission, "Counter Attack," to win Dark Magician Girl.
Enter a battle with Dark Magician and Dark Magician Girl in your party. When it is Dark Magician's turn to attack, you will be able to select the "Dark Burning Magic" attack from his battle menu. This works with either Dark Magician.