You need to play daily and watch full battle and remember opponent's best teams. Second step is one by one you have to try every remembered team so you will get idea to use some best monsters for your next battle. Now come to the third step, Level up your favorite monster along with the summoner to increase strength. And of course understanding opponent's previously used team and battle rule are important. Collect your favorite monsters and useful monsters for every battle rule. This is how you can build perfect deck.

And if you want some cards to give on rent then give only extra cards on rent that you are not playing.