To do a rapid cancel you need to have 50% heat and during the combo input A+B+C at the same time, it will make a red circle around you saying you did a rapid cancel.
An easy way to do them is with the character Hibiki, when you do forward + circle it will do a kind of combo going in the air, during this combo do A+B+C at the same time(square+triangle+circle with default button).
Repeat until trophy unlocks, can be done with anyone but easy to do with Hibiki if you focus on this trophy