I beat the game last night and here is my tip to beat Nash and all the other Heavyweights.
I found that the default grapple (hit Y), dropping chin/neck breaker, was pretty much unblockable. So I was able to get Jarrett on the ground with the default grapple, then either kick/punch or do a quick submission.
As soon as Jarrett got up I would do the default grapple and then continue the punch/kick or submission until his body was red and then used a torture rack to win the match.