I must agree with many of the previous comments.
I just spent about 5200 kobans on refills and nothing. Thats approximately 480 fights against the dark lord not counting the ones I played since the event started. Even without the supposed increasing of the chance, if you do not getting her, thats less than about 0.2% drop chance and that's a reaaally optimistic guess.
Well, I guess I will just go and loose every fight from now on in the arena against others who managed to get her thanks to their much better stats. Yeah... great event... spent all my kobans and a bunch of time on basically nothing. So much fun.
At least pleeeease remove the level cap on the dark lord so the money we get is not so atrocious.
(I wouldn't be terribly surprised after this gargantuan jump to pay to win model if her drop chance would have skyrocketed if I actually spent real money to buy kobans...)