That was kinda funny because it says you replied in June, but I just got a notification today!
In any case, yes it may be that Shankaracharya refuted the Buddha's teaching, but I am not sure that there is valid evidence that he was the one that caused the decline of Buddhism in India. As far as any text refuting Shankaracharya's views, Nagarujuna had already done so about 600 years prior by correctly understanding the view of the Upanisadas regarding an existant self. Many of the schools of Vedanta were flourishing at that time. Perhaps Shankaracharya repackaged things a bit.
It is also my understanding that Gaudapada himself altered the view of Advaita by mixing it with some views of Nagarjuna. This is what many scholars point to. But don't ask me to explain the differences because my understanding is a bit rough. You can google it I am sure.