A prime Fedor would give Cain a run for his money. Fast hands, great submissions, good wrestling, great cardio and most of all Fedor was tough as hell. As tough as about any fighter ever, it took Bigfoot Silva laying on him punching him dozens of times directly in the face to really soften him up. Cain is not a power puncher and Fedor would be able to withstand the punches of Cain, and Fedor was about as good as anyone on the ground. He was in his prime 10 years ago when he fought Coleman, Randleman and Nog. Put that guy against Cain and he'd stand a great chance, the Fedor of today is worn down from the abuse he suffered at the hands of Bigfoot, Crocop, Nog, Randleman etc. etc.
Cain not being a power puncher and Fedor being as tough and fast as any heavyweight makes it anything but an easy fight for Cain. Seriously, back in the day you could hit Fedor with a baseball and he'd still be standing.