I'm gonna use an Xbox Series X controller in this example. I'm not sure if you can do this on KB+M.
Basically what we want to do is to edit the settings of the A button and RT button on the Xbox controller and add Turbo Toggle to them.
The A button is medium attack on modern controls and the confirm button in menus.
RT is the combo assist button on modern controls.

Change the settings for both these buttons like this:


What do these settings do?
Once you press the button it will keep repeating the input until you press it again.
So you're gonna have to change these settings back to normal once you're done farming.

Once you've prepared the turbo buttons boot up the game.
I recommend you navigate the menus with KB+M.
The turbo buttons on your controller will make it pretty difficult otherwise.

• Go to Fighting Grounds and start a 1vs1 match against the CPU.
  
• Set the Round Count to 1
  
  
• Pick a character (a character with an advancing normal on the Modern Controls medium attack is preferred like Juri).
  
• Switch to Modern Controls
  
  
• Give yourself the 5 star advantage
  
  
• Set the CPU level to 1
  
• Press both Turbo buttons on your controller on the loading screen.
  
• Your character will now continuously spam their medium auto combo and instantly rematch without you having to do anything.
  
  
• And that's all there is to it!