This trophy is titled "Animation Conservation" and it's for using rapid cancel 100 times. Rapid cancel is initiated by pressing A,B and C (Square, Triangle, Circle) at the same time after landing an attack. It will make your character flash and a red circle will appear, you should also hear the announcer say "Rapid" when you do this.
The best choice for this achievement is with a second controller or another player. Your character of choice will be "Unlimited Hakumen" which is selected by pressing L1 while hovering over him at the character select screen. The only problem is you must first complete arcade mode with Hakumen to unlock this form. The trophy is still possible with a standard Hakumen, only it takes much longer given the fact his meter fills slower. It's also worth noting that instant blocks increase the meter as well, these are initiated by pressing back just after attacking with your second controller. The meter fills relatively quickly on it's own without having to do this when selecting "Unlimited Hakumen", but if you want to reduce the time some that is the way to do it.
Each rapid cancel takes half of the meter, so allow it to fill and attack with your primary controller followed by an immediate input of Square, Triangle and Circle all at the same time. This is also set to L2, however I personally found the timing trickier when pressing this instead of the 3 buttons. This is a little bit of a grind, but nothing too mind numbing. The trophy should pop immediately after you execute your 100th rapid cancel.