I favor Russell but Marnie punked Eric bigtime. Mary Anne's blood was poison and her claws paralyzed you as well. Mary Anne also knocked down bullets out of the air and almost seemed to laugh off anyone who even attempted to screw with her.
Marnie would only be a problem for Russel if he gave her enough time to start chanting. Mary Anne is only a problem if he bit her. I never really did get why Eric and Bill didn't just pummel her with fists... they had to go and try to bite her.
So, Russel still overall. Marnie vs. Mary Anne is harder. Marnie has way more variety in her power (like shields and stuff) but she needs time to make them. Mary Anne's magic is near instantaneous. So if it was a random brawl, then Mary Anne wins. Given prep time, Marnie can beat even Russel.