Time Changer is the Supermove of Flash, performed by pressing
+
when your super meter is full at 4 bars. It's perhaps my favorite supermove, which you should experience at least once.
Anyway, easiest way to earn this is in either Versus or Single Fight on Very Easy. Pick The Flash as your character, then pick Captain Cold as your opponent. Whittle his health down to 25% or so while using super moves to fill the meter quicker, then perform the move to KO him and the trophy will pop.