I agree that upgrading your summoner(s) is critical to progressing in this game. Even if you win common cards and combine them into stronger cards, they will do you no good if you do not have a high enough summoner to use the upgraded cards. This should be your first priority.

I have managed to get four upgraded summoners (in different factions) and have noticed that, although I am still learning the best cards/combinations to use, my win percentage has increased (it doesn't take me near as long to complete my daily quest).