I need to travel from HK (Wanchai) to Mission Hills Dongguan.

You are suggested to take Island Line and transfer to Tsuen Wan Line at Admiralty. Then transfer to Kwun Tong Line at Mong Kok. The last transfer to East Rail Line at Kowloon Tong, get off at Lok Ma Chau.

After going out of the subway station, you need to pass through the Futian Port.

Then, take subway line 3 and transfer to line 1 at Laojie. Get off at Luohu from Exit C. Then you will see Shenzhen Station, from where you can take a bullet train to DG.
The train takes about 40 minutes and CNY58.5 for a first class seat, CNY53.5 for a second.

Finally, take bus 861 and transfer to 847 at Huangjiang Square, and get off at Tianxin Market. Take a taxi then, which takes about 20 minutes and CNY40-50.