She dominated Saurfang and Bolvar in each fight. She one shot Saurfang with her shadow magic and she utterly dominated Bolvar. The guy couldn't do anything against her.
Now some people will claim that she is not a Mary Sue, because her new powers are explained to originate from the Jailer, since when he grows in power, so does Sylvanas.
But that only proves that she is a Mary Sue. Mary Sue's always have justifications for their powers. It's just something the writers pull out of their ass.