Instructions
Play through the game's Story Mode five times. Sadie will only duel with you once you have beaten the game on five separate occasions, from beginning to end -- you can't simply play the last chapter of the game five times.
Pick a different Tier 1 partner each time you play the game. Once you beat the game and start over from the beginning, you can pick a new partner each time. The Tier 1 duelists are the ones that appear on the first page of your duelist log. These duelists are Jaden Yuki, Syrus Truesdale, Chumley Huffington, Alexis Rhodes, Chazz Princeton, Bastion Misawa and Zane Truesdale.
Go to the shop at Duel Academy after you have completed your fifth playthrough. Speak with Sadie and she will duel with you.
Use your most powerful defensive cards when dueling against Sadie. The duel is essentially like any other, though her cards are fairly strong. Even though you will be able to duel her as soon as you restart the game, do not underestimate her. However, after having played the game five times, you should have amassed a strong deck -- how you defeat her depends on your style of dueling.