Brock easily took Cain down in their fight. Once he got slapped in the face he cowered away and became less aggressive. He couldnt hold Cain down.
I have no doubt he could take down any HW fighting today. Keeping them down is another matter. But if he is going to wince at every punch he wont be elite. In saying that, I would say he would have a chance if he could Cormier down before being hit. Other than that id say it would be a flash KO in the first round.