To get this, you can do any game on Hard difficulty in any of the modes where Bonus XP is offered: Classic mode, Events, Star Club or Daily Challenges. However, this seems to be buggy as it unlocked after I finished a medium puzzle and got 1000 XP points (500 x 2). It appears that once you do a puzzle that offers at least 1000 XP (500 x 2 due to the bonus), this will pop.