Dropping your starter attack and defend cards after getting better alternatives is a good tip for new players, but the number one factor in winning in my experience is selecting the right card combos and synergies. Identify possible combos early and pick future cards to build on those strategies. If a card doesn't fit in your deck skip it. Sometimes you'll pick a combo card early and not come across any synergistic cards and lose or have to pivot strategies, it just happens. Having a smaller deck with only the cards you want will usually help though.