You know, I can't remember what I did to hold the clutch plate from turning. I don't think the 12mm was all that tight when I worked on the compressor. How about a strap wrench, like for a spin-on oil filter?

BTW, here's a link to a site which explains rebuilding a nippondenso compressor. Most of it isn't relevant, but some of the photos & steps might apply to your task. http://www.pelicanparts.com/techarticles/911_Nippondenso_rebuild/911_Nippondenso_rebuild.htm

Best of luck,

- JimY