Use the auto-save to save scum. Might depend on your platform, though. On PS4 at least, you can close the game at any time in the Tower, and then restart the game and select “Continue” from the main menu, and you’re brought right back into the Tower after the last battle you finished. You can even close the game on the “You lose” message screen showing your point total, and load right back into the Tower just fine. As long as you don’t go back to the main game, your auto-save will always be in the Tower.
For a more honest method, just be aware that you can run away from the boss battles with 100% success rate. This seems to be a deliberate design choice, as there would be no other way to configure your party strategy for the mid-bosses, since they are all random. So if things are suddenly going bad in the fight, just run away, re-do your strategy, and try again. Or just keep trying until you get luckier one time.