What is the best way to get visa? Also I need to get one for my assistant too.

The best way is to let a professional agency to deal it for you. But you need to pay them service fee. If you don't want them apply for you, you need to apply for the M type one on your own. You need to provide the invitation letter from your Chinese partner and detailed information about the inviter and other documents required by the embassy excluding your passport, filled application form and photo.