Tips to building good decks.
1) Try to keep your deck size to a minimum. The smaller the deck, the better chance you have of drawing any particular card you need. For example, if you have three "Aisha Myriad" cards in your deck, and you really need one, it's better to have a 3:45 (1 of 15) chance of drawing it than a 3:75 (1 of 25).
2) Pick a theme and stick to it. Find cards that work together well, and make your deck around that theme. Don't put cards in just because they're good cards. Only use them if they work with your theme.
3) Playtest a lot. Play as many games with your deck as possible, and against as many different decks as you can. This will help you recognize which cards you can get rid of, and which cards you need to fix problems.
4) Modify your deck for the local trends. Find out what type of deck most people are playing and build your deck to beat it. Do this by learning the weaknesses of that type of deck so you can exploit them to the max.
5) Make the deck fun. If you don't like playing it, you won't like playing the game. Play a deck that you'll have fun playing, even if it means it's not the most powerful deck out there.
6) Be original. The more original you are, the less likely it will be that someone can specifically build a deck to beat yours.
7) Practice. The more you practice, the more likely you are to notice new combinations within your own deck that you previously didn't think of.
8) Don't be a sore loser. When you lose, instead of being a crab about it, figure out why you lost. Did you have a bad draw? Maybe you need trim some cards out that aren't helpful. Did you simply get overpowered? Maybe you need some more/better equips, or something that gets rid of opponent equips/exp pets. Did you not get your key cards quickly? Maybe you need to thin the deck and add in some cards that let you search your deck for what you need.
9) Let your friends take a look at your build. They may be able to suggest a card to add, a combo, or point out a weakness.
10) Trade. Just because you don't have a card that you want in your deck doesn't mean you can't get it. Remember those good cards that didn't work with your deck idea? Trade them for good cards that do. Usually, you can work out a deal with a friend to help them out and help you out. Another option is to simply buy the single cards.
These are just a few ideas to get you on your way. I hope they