I planned to visit Shenzhen via Hong Kong just for a day

I planned to visit Shenzhen via Hong Kong just for a day arriving early Saturday morning then stay in a hotel then leave back for Hong Kong early Sunday morning then back home to manila evening time.

Do I need a china visa just for a day in Hong Kong? If so, is it better to apply theVOA to Shenzhen after the bus ride from HKIA? Is there a guarantee for a visa and how much is the fee for applying at shenzen? thanks