Instructions
Travel to the closest shop. Stock up on repair powders, water and food. You will need a good stock so that you can continue to battle enemies without returning to town.
Enter the Demon Shaft dungeon. Where you start leveling Ruby will depend on how much you've used her during the game. If you've barely used her, start at the first floor of the Demon Shaft. Most likely you'll be able to start around the 40th floor of the Demon Shaft.
Continue clearing out floors with Ruby. As with any other character, keep her bracelet repaired (with the repair powders you bought in town), because if it breaks you will have to start over with a new bracelet.
Finish the 99th floor of the Demon Shaft with just Ruby. It is important that she clear the entire floor on her own. Now you should have a strong enough bracelet to defeat the Dark Genie.