You have not informed your planned travel date. From Hong Kong to Guilin, there are two daily direct flights and some connecting flights for your choice. Flights information:
Departure Time: 16:05, Arrival Time: 17:35, HX121, Hong Kong Airlines
Departure Time: 19:35, Arrival Time: 20:50, CZ3032, China Southern Airlines

Take September 20, 2009 for example, the lowest airfare for CZ3032 on that day is CNY1,460 excluding the tax of CNY167.

From Shenzhen to Guilin, there are three or four flights available daily. The earliest flight is at 09:00 and the last one is at 20:25 or 21:00. Flight duration: about 1 hour. Full airfare: CNY660. Many discounted tickets can be brought.
I suggest you take the second way: Shenzhen-Guilin for this will be more economical. Traveling from Hong Kong to Shenzhen is rather convenient.