It's just accounts with level 1 cards playing against other accounts with level 1 cards over and over and slowly but surely moving up the leagues.
That's not true in my case. Nonetheless, what about this? Each game has a pot based on the total level of cards played. So level cards would essentially lower the pot. The winner takes the whole pot. It encourages players to level up because 1. It's easier to win with leveled up cards. 2. A player with leveled up cards will earn more over time.
This rewards good play which will make the game more fun and allows a mechanism to reward purchases.