I'm under the impression that Half Bloods are generally regarded as not worth farming. You need some upped cards to really stand a chance, the battles take quite a while, and the rewards are minimal for the effort.
If I were you, I'd build a golem rush deck and farm Top 50 - you have an excellent chance of running into a deck that's built with the express purpose of winning you some rare cards. Between that and farming AI3, you should start working toward a False God deck.
Here's a couple T50 decks you can use unupped:
http://elementscommunity.org/forum/index.php/topic,11026.0.htmlThe second deck listed there uses Arsenic, which is a rare weapon. If you don't have one, swap it for an extra Nova or perhaps a Hammer.