In reply to the OP, I can vouch for ScaredGirl's advice. I was running a Firefly Queen Deck with several upgrades earned the hard way - grinding away at Level 3 since I had no chance against gods. Then, one day I finally opened up this forum as you have and IMMEDIATELY sold all of my non-rares and even a couple of my upgraded towers (since I knew they'd be eas-ish to replace) and used the money to buy scaredgirl's scarabs deck. I've since tried several others, but only because I using one of scaredgirl's decks (mostly unupgraded, to boot!). Even without upgrades, it allows you to start winning at least 25% of the time. And then you get three spins to win an upgraded card (the only kind the false gods use). If you can put it into your deck, do it, otherwise, sell it and save the money. Upgrade your towers first, then prioritize the cards in the deck for upgrading. The more upgrades, the better the winning percentage. You'll also get better at playing the deck through experience and make fewer mistakes, but go ahead and read her and others' advice about how to play whichever deck you choose - it'll make your life easier. Good luck!
-TheArchduke of Elements