I'm copy-pasting this from another thread.
I wrote it here:
https://steamcommunity.com/app/850980/discussions/0/1638661595066114603/

Made my own thread about it here:
https://steamcommunity.com/app/850980/discussions/0/2733047810447262411/

and thought it'd be nice to share this information here just in case.

AND this strategy is not my own original strategy. I used the one I found here:
https://forum.psnprofiles.com/topic/74922-guilty-gear-100-completion-hints/

1. Go to the options and set the game timer to 30 seconds. Trust me, you will need this. A turtle victory is still a victory.

2. When you start up normal mode, have a second controller plugged in and pick Sol Badguy or Ky Kiske. As you go through, feel free to Insta-Kill the first 4 opponents (these guys are basically flukes, it's not until level 5 onward the difficulty becomes adaptive).

3. Whenever you're about to lose the match, immediately press start on the 2P controller. I recommend doing this whenever your Yellow Health is half drained, so as to not risk a loss too much. This will pit you in a versus match. Win this match and you will put back into the arcade match you were on beforehand, now given a second chance. Rinse and repeat until you beat Justice.

4. Once you defeat Justice, you will see the ending and credits. When the credits end, you will fight Baiken. This is where you can relax a bit. With Baiken, you can continue all you want until you defeat her.

5. After beating Baiken, head to Versus mode. You can now play with Testament, Justice, and Baiken. Have fun!