If you want to get better cards, I suggest saving your DP. You can get decent DP from the following ways(should always work)
1.Dueling Tea(uses alot of cards that brings her LP up, you get DP also based on amout of LP damage you deal)
2. Spending little to no money from tournament(Weekend tournament = 1000 DP total. Limited Tournament = about 300-700 DP per opponent. K.C Cup= varying depending on round the more into the tournament you get the more money you get each time.)
Ive gotten all 5 exodia pieces 4 times easily, but it does require patience
**SPOILER** I got the Exodia pieces from the Super Monster Pack(500 DP per pack and 5 cards per pack)
This should really help your game, as well as skills
If you're having trouble beating a certain opponent in a tournament you could just restart your GBA before it saves ur lose.(best time would be when it says "You Lose")
I know its simple right? You should already saved the day of the tournament once you start up your GBA again it should take you right back to your house and it should still say the name of the tournament by the door when you go out.
An easier way to get DP is if you keep dueling Tea.
I know it sounds boring but u also get DP dapending on how many Life Points you deplete from your opponent. Tea uses alot of cards that increases her Life Points,(such as Nimble Momangan which when its destroyed and sent to the graveyard she gets 1000 LP plus 1 or 2 more of them to her field which means 2000 more LP when you destroy them.)
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.