I always use one of these Sugino chainring bolt tools whenever tightening, un-tighteneing etc.
Fits in and stops the back of the bolt turning. Only cheap online too. Saves using a screwdriver or pointy object, which usually for me ended up slipping and stabbing myself or scraping the chainring/spider/powerdisc.
If you get your hands on one of these you should find the bolt will undo quite easily. Even if you have to tap a screwdriver into the rounded out allen key holes.
G.
