Heres a tip that will help you save your points.Dont waist your points on the Guardian cards(for ex:Guardian Kay'est)Those cards are not in the game.Sorry Guardian card fans.

Affter you unlock the Rare Hunters shrine go there on Thursday and look around for Bakura he will be there and you can duel him when he is wearing the Millenium Ring.

Here are the codes for two very powerful cards.
Black Luster Soldier-Envoy of the Beginning-72989439
Chaos Emperor-Envoy of the End-82301904
Hope you like these cards and the best time to use them are in the Rare Hunters shrine.Also you can only enter the codes in the Rare Hunters shrines card code machine.

Duel everyone inside the card shop on Monday.After you beat them all inside the card shop talk to Grandpa Muto and you can enter a tournament on Tusday hoasted by Grandpa Muto.

If you finish the game then at the title screen use this code press Up,Up,Down,Down,Left,Right,Left,Right,B,A to change the title screen to blue insted of purple.Repeat the code to disable the effect.

Put a Time Wizard in your deck and talk to Joey and he will duel you using Time Wizard.

If you made your own Exodia deck then go to the card shop and talk to Grandpa Moto and you will duel him using an Exodia deck.