Really? I don't think Randy will be able to deal with Lesnars strength and speed. What kind of submission do you think he will catch him with?
Don't know, there's hundreds of possibilities. I wouldn't be surprised if it's a leg attack, but it won't be as easy as what Mir pulled off with that kneebar. The problem with Lesnar is that while he's a freak of nature, he hasn't put in the time. Having a lot of money and being able to train full time will bring your game up in a hurry, but that's not enough to deal with the experience of somebody that's been doing submission grappling for over a decade, and runs one of the top pro gyms in the country that's loaded down with talent.
If it stays standing, Lesnar can use his size advantage. Unfortunately for Lesnar, his striking kind of sucks, so he has to take it to the ground and do nothing but stay on top and try to pound him out through his guard. Being in somebody's guard is not where you want to be. Also, a 280 pound frame needs tons of oxygen to keep functioning, so he'll burn out faster that even an aged Couture.