During high season there is at least one bus that goes straight from Nanning to Detian. 8:30AM from Langdong bus terminal on Minzu Dadao, ¥50.
During low season there are buses going from Nanning to Daxin on a semi-regular basis. In Daxin there is a single bus from the bus station that goes to Detian at around 11:20AM, though again, only during high season. During low season inquire at your hotel how to get to the waterfall. If you can manage to tell them that there is no bus from the bus station they will direct you to where 12-person vans regularly depart for Xialei ¥12. If you tell them that you are going to Detian they will drop you off in Shuolong. From here you can easily find a smaller 6-person van that will take you the rest of the day ¥3 though they may try to convince you to pay more if they are having trouble finding additional passengers.
