It's a traditional-style Torx chainring bolt, and the two notches on the 'nut' have been damaged in my attempts to unscrew the bolt. Never actually had this happen to me in just over 30 years of cycling.

Drill it? But won't the thing just spin with the rotation of the drill bit?

Carbon fibre Stronglight Fission crankset so really do not want to wreck it.

Any advice would be most appreciated. Is there a trick that I do not know?

You can soak it in some penetrating oil that will make it easier to turn.
Then you can use a dremel to make 2 new notches in the nut to help you turn it.

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