I had a similar problem, I could not tighten one of my chainring bolts even using the park tool. I finally took the bike to the LBS and they had a better version of the tool, shaped like a screwdriver. With one person holding the tool firm, one holding the bike steady, and me tightening the chainring bolt, we got the job done!

Any links/leads to the screwdriver type tool? All my searches lead to the standard type...