Nightmare boss rush? Just leave the system on as long as needed, lol. Your preset levels levels are pretty low. Knowing patterns and infinite continues are your best friends.
I would say level 48 at the lowest for end game Nightmare. I would say it gets “easy” by 53-55 and it shouldn’t be too hard to level up to that point, even on nightmare. One or two levels can be the difference between getting stomped and victory.