Multpile Medium Term Stays in China

My long term gf of over 4 years lives in shenzhen. I went to the visa service centre in london and they told me i could not get a multiple entry one to see her. At best I could get dual entry, but in addition to that i would have to apply in hong kong not in uk because I will always be at sea for more than 3 months before the date of first entry. Do you have any advice on how i could stay in shenzhen twice a year for 3 months at a time? The london service centre also never mentioned i would need a letter before going to hk to apply for dual entry, what is meant by an appropriate authority?