Carwin isn't going to win a wrestling match with Lesnar. Lesnar is a freak, plan and simple. Carwin is a big dude, but he can't hang with Lesnar's size and even speed.
Carwin could knock Brock out, but then again, Mir could have knocked Brock out. You have to get outside of your comfort zone in terms of range to hit Brock, and Brock is a heavy that can shoot across the mat in a partial second to take down his opponent, now try and get into striking range with him. Big trouble for anyone.
Funny as this may sound, a guy like Tim Sylvia would have the best shot, because he has the reach to tag Brock.
Fedor, he has such good hips on the mat, especially on his back. He moves so well, unlike Mir who was just waiting for Brock to put himself in a bad spot, Fedor will actually work for position, working three moves ahead.