Just a terrible event.
I don't know who created the "scalable" drop rate formula for these events depending on duration, but it needs redone.
I never once capped on combativity the whole time, only fought dark lord, spent over 5000 kobans on refills, and nothing. I don't know what more I should be doing but it isn't fun and goes against the spirit of the game.
The reason I play the game over others like it is that there is a valid free to pay method as long as I grind, but with this event the formula clearly hadn't been adapted for the grind to be valid like it normally would be. I don't mind spending a few kobans, but that's just stupid, especially not to get her in the end.
If she'd been a pachinko girl that would have been fine, but it's on dark lord of all bosses, so I didn't even make significant money from it.
I also don't know if event girls chance is based on the number of girls on your harem as well, but if it is scrap that in future, it's a terrible idea that makes your game terrible to play.