You shouldn't expect it to be easy or earn much but if you play consistently in Ranked and Frontier your collection of cards will slowly grow and improve.

As you get more cards you will have more choices for all the different matches and perform better.

At the same time you need to make the most out of what you've got, right now you're still making some mistakes that are easy to be fixed in your lineups.

One very popular strategy is to use the Frontier summoner that improves magic attacks.

I really don't know how effective it would be for you to rent cards, if I were you I'd just buy cards with magic attacks that you're still missing one copy of for the neutral and water splinters, start from the ones that only cost a few cents.

Torrent sniper. mindless thrall, abyssal elemental and shock trooper shouldn't cost too much and they will help you win more games. That way you'll earn more rewards and slowly build out your collection.