Yu-Gi-Oh! Worldwide Edition: Stairway to the Destined Duel Cheats

Never Lose a Duel
If your life points are nearly zero, or are being reduced to zero in a duel, simply switch your GBA off. You will not have a recorded loss. (This also resets the day's announcements.)

[Suggestion: lose (only) your first 5 duels so you can unlock Mokuba; that way you don't lose any points.]