The way to do a comeback is that you have to defeat the opponent on Turn 4 or later while having less life.
So what I do is:
1. Load the settings (jp.konami.duelinks.txt) in GG for the 8k assessment.
2. Click “My deck” and set value to 20 and unfreeze the value
3. Click “Enemy Deck” and set value to 20 and unfreeze the value
4. Click “Card in Hand” and set value to 4 and unfreeze the value
5. (optional) Click the Down arrow to save the file as a new file, in case you want to reload this for future. Give it a new name.
6. On Turn 4 or Turn 5 (depending on whether or not you go first), open GG again and set the “Enemy Deck” to 0.
7. End Turn on Turn 4 or Turn 5 after making the change above.
So this way you start the game normally. But you cannot die because your life is frozen at 1. You can survive until Turn 4 or Turn 5 and then deck out your opponent the turn after. On the assessment screen it may not show the comeback victory since you get a “Deckout” victory. But on the mission progress, you’ll see that comeback victory increased by 1.