Win at least 20 matches on Hard difficulty or higher in Universe or Play Mode (Single player).

Make sure the match difficulty is set to hard or higher, and win 20 times. I suggest giving the Superstar you play as infinite finishers, as well as turning the K.O. option on. After 4-5 finishers your opponent should be knocked out. Hit rematch until you've reached your goal.

Tip: Use Big Show as he has the Knock Out Punch. It makes things a bit easier. See this video if you're having trouble