Select 'Battle' then 'Versus' with the following settings:
Player: Player vs CPU
Round Count: 5
Battle Time: Infinite
CPU Difficulty: Easy
Battle Mode: Standard
Pause Menu: Standard
Select any characters for yourself and your opponent. In the match to perform a Reversal Edge press Y+A to activate it and if it lands you and your opponent will go into a slow motion effect at which point you should press X, Y or B to land the winning blow. If it is successful then you will beat your opponent at which point you can repeat the process.
Whilst performing these Reversal Edges you will build up your Soul Charge. When it is full, activate it (Hold Left on D-Pad + RT) and continue performing Reversal Edges and repeating the process. Doing so will also unlock:
This can also be done against a second controller but on Easy this is still very simple to complete.