This should only take a few fights. For the example I will use Raiden:
In options change the no. of rounds to 1.
Select Single Player and single match. Select Raiden and any opponent. Change the difficulty to very easy and select any stage.
Complete the fight as normal but when the opponent has very little health do the following:
Stand next to him and press
and then keep hitting
until the opponent explodes. It will say "BRUTALITY" on the screen.
Keep hitting rematch until you reach faction level 5(it's the number on the right of your progress bar)
This can with 2 controllers