Good question. With the new False Gods and the new AI changes, I'm not really sure. Hope/Ray of Light seems promising. This type of Rainbow build is always popular, but there are so many versions that it's difficult to say which build is the best. I use a rainbow build simply because I have 13 Quantum Towers and I don't have any other type of upgraded pillars. If you don't have the upgraded cards, then Scaredgirl's almost non-upgraded rainbow is best, but if you have the means to upgrade cards, then there are many different deck types that should do well against FGs. You might want to go to the trainer and build several different fully upgraded decks and test them out to see what works. In fact, I think I'm going to take a break from fg farming and go over to the trainer myself. I want to try out a couple deck ideas with upgraded cards to see where I go from here...
Peace,
Jallen