Fedor has some quick hands for a big heavyweight fighter. I still think he's highly over-rated over the last few years.
At one time I agreed he was the hands down best fighter in the world at HW. But he hasn't fought anyone worth a crap in a while now and his last 3 wins are over guys that I seriously don't believe could beat any of the top 3 or 4 HWs in the UFC at this point in their careers.
That said you can't count Fedor out cause he has punching power, takes punches well, and he has those quick hands. If he and Lesnar faced there is every bit the good chance he'd KO Lesnar.
However if they went to the ground then Fedor would be in trouble regardless off how good he is on the ground because of the enormous size and strength difference he'd be giving up to Brock. If Brock could get him in that chest to chest position on the ground it would be very difficult for Fedor to do much about it.
That said as much as I'd love to see the fight, and I'd love to see Brock smash him, I'd probably say that Fedor would keep it on their feet long enough to KO Brock.