As you progress through your career you will be issued 'Throwdown challenges' by the Superstars. You'll hear them speaking to you...something along the lines of "How about we try that again...one on one?" or something along those lines, then you'll see an icon of their head showing at the bottom of the event card for that location. When you're ready to accept the challenge, click their head. Win 12 of these and the achievement is yours.
This achievement can be earned on ANY difficulty, even easy.