How to marry a Chinese woman in Shenzhen?

Hi, there is no agency that can help you with this. You and your partner have to present in person for registration of marriage in her local Civil Affairs Office (民政局). Document needed include: Your passport, temporary registration of residence permit after entry (this one is issued by police station or through the hotel you will stay), and a Notary Certificate of Single Status done in USA (An Affidavit of Single Status signed before a notary public, certified by the clerk of Court of the County in which the document is commissioned, then certified by the Secretary of the State, and finally certified by the relevant Chinese consulate). Your Chinese partner need to provide her Hukou book (户口本) and ID Card (身份证) and a photo of you both taken together.
10 day visit is enough for this if you have all these documents welll prepared.