Booster packs are unlocked through various tasks and objectives within the game. Some can be unlocked through progessing in the Story Mode. Others are unlocked by completing tasks in the game. However, most are unlocked by beating various Duelists three times each.
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There are various different ways to unlock packs. Try beating everyone you come across 3 times. That should unlock a lot. Some can only be unlocked through story mode. Look on gamefaqs.com. That should tell you more.
You have to but the Super Polymerization Overdrive Pack. It features both Yugi's and Jaden's cards. TO unlock the pack you have to get 30% complete card packs from 49-57.
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You get to level 22 and beat Yami Marik Then you will have all packs available however some cards you have to trade to get.