Why are the drop rates so low? I suggest that you double the drop rate to 5 soulstones per 3-sweep. From Chapters 1 through 16, the soulstone drop rate for each sweep gets gradually higher. From a 3-sweep average of maybe 2.5 soulstones in Chapter 1 to around a 4.5 average in Chapter 16. This is a nice touch because chapters get more difficult and we have to pay more stamina per sweep. But somehow, Chapter 17 has the same lousy drop rates as Chapter 1. Why is this?
It would be great if you could address this. After all, Sarya is not available anywhere else and getting her to 7 stars at this rate would take more than a year if you farm daily!