So, farming the girls currently is not really fun but rather frustrating. You fight a boss again and again and on most days, you get nothing as the drop rate for a girl is really low. Now, I get why the rate is that low as players should be motivated to use the epic pachinko. However, grinding all day without gaining anything is frustrating and no fun. To solve this, I'd suggest to split a girl into multiple pieces.

Let's consider for example, that the drop rate for a girl is 0.05% (I have no idea about the real values, but just picked one low value). Now, on average, you get one girl during 2000 boss fights. Now, let's split this drop into 100 pieces that drop with a probability of 5%. When I have gained 100 pieces, I get a girl from the corresponding boss. In this new scenario, you would not have those frustrating days without anything but earn some pieces every day (or at least most days, bad luck is still possible) and feel like you achieved something. Maybe you're even happy about a lucky day when you earned more pieces than normal. When considering the average, you still get one girl every 2000 boss fights but removed the frustrating days of achieving nothing. The thing that changes however, is the variance. Thus it will be less likely for people to get their girls after just a few boss but also more unlikely to wait a lot longer than the average.

I think this method is easy to implement and would really help with the motivation during farming girls, as you can see your advancement instead of hoping that today is the day, which it usually is not and you've spent a whole day without achieving anything.