It is important that you have a balance between cheap and efficient cards and expensive powerful cards. Your deck of cards also needs to have a balance of cards so you are able to counter several different units from the opponent. Also make sure you don't put all your cards containing high aura in your deck as you have to wait for aura refils in order to summon them which will put you at a disadvantage during battle. Get in the habit of checking the card details for every character by clicking on the info of that selected card.