As you all know, (If you read my profile) I have an interest in the RFI, and I think I've found an answer to how Nix uses the RFI against The Beast. Wolfe designed the RFI to cure the plague; Cole wanted to kill The Beast; the RFI's normal function is a sort of ray, like what Nix did; This cure's the plague, but kills all those with the Conduit gene. Wolfe knew Cole couldn't and wouldn't go around the world, firing the RFI at every human on earth; so, he developed the RFI to have a bigger radius; the world. Unfortunately, this needs blast cores to power. So Wolfe just told Cole this would weaken The Beast; he didn't reveal it killed all conduits. So Cole did Wolfe's bidding. This explains why Nix couldn't just activate the RFI; she needed to absorb the blast cores. But she didn't. Cole did. Cole was the only one who could fully activate the RFI. No, this isn't fan fiction; no, this is not a fact; it is my simple opinion. I've just wanted to get it off my chest. Thank you.
How the RFI works
You all probably know this, but it took me a while to understand it. Wolfe says the RFI is an anti-ray sphere; he was hinting to Cole as to what it would do. The ray sphere drains the neuro-electric energy from humans and transfers it to conduits; the RFI drains the neuro-electric energy from conduits to humans, doing what the ray sphere does; killing humans to power other humans, except this drains conduits, which kills them like the ray sphere kills those without the conduit gene. Just wanted to add that.