You can do this in any mode and will probably get this by accident simply playing the character.
However a simple way to insure you get it is play a CPU match on the NOOB difficulty or local match with a second controller and keep spamming (quarter circle forward punch, I'll show the button input which will assume you are on the left side of the screen and using default controls)
+
+
+
/
/
. (Those are all the different punch buttons, you can use any of them but they'll give different effect to the attack.)
Every ball that hits will count one to the counter and once you hit them with twenty balls it will pop in the middle of the match.