My suggestion is simple. Make Passive Tumble/Passive Flip:
A: Take a small amount of balance. I'm wary of this, because it certainly doesn't feel nice to have an passive that eats resources. However, balance regeneration should fix it fast enough. It would just slow down Duelist balance buildup a little bit, even if it's losing an average of, maybe, 1.5 balance every twelve seconds.
B: Have a minimum balance. Something low, like -10, because Rias has said that he wants Duelists to be 'Balance Masters'. I find it strange that one can Tumble away from strikes at -25 balance. It just feels that, if I'm able to be patient enough/roll well enough to overcome a Duelist's defenses, I should actually get to run with it a bit.
It's been a long time since I've played Duelist, and I've still never played Duelist above three hundred and fifty melee. Someone with more direct experience than me might be able to explain thoughts upon this front, though I've considered these thoughts rather hard and determined that it might make Duelist combat both more rewarding and thought-based, and also less impossible to deal with from the other side.