you could grab that with a pair of mole-grips, get it down tight 1st time in the rounded part of the grips.
Use these but pinch the bolt head on, really tight, and unscrew. (so you turn it like a screwdriver) these grips have save me many a time.
@TM started
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Use these but pinch the bolt head on, really tight, and unscrew. (so you turn it like a screwdriver) these grips have save me many a time.