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Morning Steevc; the cards are really really much driven by brawls. I've come to play the following strategy:
Do my daily quest (if it isn't life or death splinter) - and then, on Brawl fights, rent out a couple of key cards - then grind to get my rental DEC back.

Kron/Llama is a very much overpowered combo, in some ways, this post was stating the obvious - you put some additional cards in, you're going to be all the more outrageous in your overkill.

Have a good one, Tim

I only rent cards out as I'm not sure I'd get value from paying for them. It's a nice little income.