You performed 20 One More Bursts in Arcade Mode or online.

To do this, I just pressed X repeatedly and during the auto combo, I pressed X + Y + B during this, and if you are lucky, youll get a One More Burst, youll notice it as youll get a picture similar to what is seen in the achievement tile come up on-screen, it has to be done while attacking your opponent, using the auto combo makes the timing much easier as you do need Burst as well to do this.