Nitro

I also guessed it half-way through the book. The more I read it the more I was re-affirmed of this. There are a few hints in the book, but I won't suggest what they are.

Well Firefight isn't exactly dangerous when you put two and two together. She's an illusionist, so she could make someone drive off a cliff, or shoot the wrong person, or cause an accident. Also, as we saw with Nightweilder, she can make a dangerous epic even more dangerous during a fight. However one on one, she has to become more creative, or simply shoot a gun at them (perhaps from behind an illusion?). She's not nearly as dangerous as other epics. However what's made clear from the end of the book is that David, and possibly the whole team, won't be trying to kill her (especially since she can't die). Also if they do kill her, her 'powers' get used to re-incarnate her. So while she may be brainwashed, she may just be as bad as any epic at that moment. So they'll try to keep her from using her powers, to turn her good. So I think the key to her downfall, is essentially stopping her from being an Epic, and letting her human side take over. How? I'm not sure. Maybe she needs sight to create her illusions (as I think it was hinted at). So perhaps keeping her from seeing is the key? The most obvious way of doing this is through emotions, memories, feelings, etc. Clearly David already has an effect on her, or else she would have killed him, and she wouldn't have been crying at the recording of him proclaiming his feelings. Therefore, I think her weakness, as cliche as this may sound, is David. That or blindness.