Fake God deck is the best way to earn money and upgraded cards, and you can win most rares, too. With that money, you can build a good deck that suits you just fine. That poison deck is similar to the one I'm using, lol. Also, AI don't usually get harder as you gain more score, you're just having bad luck. XD
If you're going with Scaredgirl's God Farming rainbow deck, I recommend Pulverizer.