With a guy like Lesnar, I think speed with enough size is the key to beating him. He isn't that well coordinated and Cain knows he has to watch out for the takedown. Cain will work on avoiding the takedowns and will probably try a lot of pot shotting on Lesnar from the outside. The cage is very big and works to Cain's advantage. I think it takes a fighter similar to Cain's mold to beat him.
I say it can go either way. They are going to have to stand up first and that advantage goes to Cain. If he can work on his leg kicks he can really bother Brock who I suspect has sensitive legs. Cain's got the cardio, striking, and speed advantage and that is going to be big. Truth be told, Brock can negate a lot of that once he gets a hold of him but don't write Cain off.