It's a hollowtech crankset, and one of the allen bolts which tightens the non-drive side crank arm is stripped. I think the previous owner put a non-OEM bolt in, since it doesn't look like the other bolt...

Anyway I tried taking a dremel to a slightly larger allen key in order to shave it down to the size of the bolt head, which still couldn't get enough purchase. Anyone have ideas on how to get it out apart from drilling it?