Instructions
Go to the art gallery after defeating Magnum in a duel at the aquarium.
Talk to Ishizu. After talking with Ishizu, you must battle her. Use dark- and fiend-type monsters to defeat Ishizu's light- and dream-attribute monster deck. Don't let Ishizu have more than three monster cards on the field at once, or she will be able to summon Obelisk the Tormentor, who can defeat many monsters and drain all of your life points in one hit.
Talk to Ishizu after the duel. After talking to her, you will be confronted by Seto and challenged to a duel.
Defeat Seto. Seto uses fiend-, light-, water-, dark- and forest-type monsters with weak attacks. Concentrate on defeating his monsters as soon as they appear on the field to prevent him from summoning his Blue Eyes White Dragon or Blue Eyes Ultimate Dragon.
Talk to Ishizu after the battle. She will give you the Obelisk the Tormentor card.