Realistically, you should take that 1500 Electrum and build one of the anti-Fake God farming decks featured on this forum, and then farm the Fake Gods for rares that can be sold for 1100~1200 electrum each, or that can be used in the deck that you aspire to build. However, I'm not going to tell you how to play this game, lol. Do it your way, but unless you want to farm AI3 and PvP for money and cards to sell for the money to upgrade, it'll take you forever. >.<"
If you want to stick with your Darkness deck, upgrade your Pillars to Towers first. I'd have to see the deck myself to know what else to upgrade.