I finally got it unlocked.
You need to complete Tournament without ever failing. What do I mean by failing? You have in each stage 5 attempts to obtain the most points. If you don't have most points in those 5 rounds, you fail. You can pick up the tournament there and have another 5 attempts to try to beat that stage. (Works to unlock all the other "tournament achievement", but fails to unlock hard mode then.
I can confirm that you can play the tournament on easy.