Business visa for China (for Swede living in Kansai, Japan)

I'm from Sweden holding a Swedish passport, and living in Japan (Kansai area, Osaka). I have a 3 year visa in Japan. I need a business visa for China.

I need to be able to go to China on short notice, at any time. Each stay is probably shorter than 30 days.
What kind of visa should I apply for? How long validity can I get if using a letter of invitation?
Can I apply by myself or do I need to use an agent to do it for me?

I read somewhere that from Mar 2012 foreigner living in Japan having a "Certificate of Alien Registration" cannot apply for a visa to China by themselves. Is it true?