Defeat the following duelists 3 times each:
Eiki, Zape, Key Mace, Key Mace #2, Gate Mace
(Gate Mace is actually hidden behind the tree
in the very first section of the forest).
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Red Dragon Archfiend can be obtained in "The Duelist Genesis" booster pack, or by defeating Jack Atlas an undesignated number of times in a Duel or Turbo Duel.
Armed Dragon LV7 appears as a Super Rare card in the "Soul of the Duelist" booster pack in "Yu-Gi-Oh! 5D's World Championship 2010: Reverse of Arcadia".
Stardust Dragon can be obtained in "The Duelist Genesisi" booster pack, or as a gift from Yusei Fudo after defeating him an undesignated number of times in a Duel or a Turbo Duel.
You must defeat these CPU inside story mode 3 times (or more) each; Eiki, Gate Mace, Key Mace, Key Mace #2, Zape.
You can't.