In the game modes select event.
You don't need to change the name or anything just make sure that you choose either 6, 7 or 8 for the fights total. You can do the auto fill with X to let the CPU select the match ups.
Save the event and then start it.
If successfully done the Achievement will pop as soon as the final match has completed.
It took me 2 tries to do this the first time I did 2 things that might have voided the achievement. The first is that I Saved the Event and then exited and started it later. The second is that I accidentally did not select a fighter for one fight and switched to control the fighter once the fight started.
Difficulty does not matter for this.