The extra bullshit in the boss stages gets harder with time. It's easier if you bring one character at a time so the stage resets when your character dies.You mean a single character at a time?
I always try to run at least two so I can swap them around and make use of healing bonus etc, but generally have a full party unless the boss is down to 20% or less health. I could also be doing it completely wrong lol
My lowest character is lvl 20 now, and I have six 40's and one 44.
I get to VS8 every day, but haven't completed it yet.