The way you do this is by healing one of your buddies. Near the end of the game you unlock all character classes and can switch them out as you see fit. Equip only healing abilities and use artifacts that increase your healing stats. Choose a brutalist like Craig who fights enemies up close. Those always take the most hits. Now simply heal him each turn until he has survived damage equivalent to twice his health bar. You can also pick other allies with healing abilities, for example Fastpass has a very strong healing ability. What you can also do, is leave one enemy alive, have Craig stand near him in the corner and just skip turns / heal with your other buddies.
Can still be done after the story (for example the hobos at the playground respawn infinitely when you go in/out of the restroom).
