Probably easier than some of these others but simply finish 300 rounds(? It's a little vague, it could be matches, hopefully not) by doing a EX move. EX moves are performed by having at least one of your small blue bars filled and then pressing two of the same type of move but at different strengths. So for example doing the motion for Ryu's Hadouken but instead of just pressing one punch button at the end you press both light and medium punch will result in a stronger fireball and deplete one bar of energy.
This can be done in any mode including local versus.