As I said, there's always the destructive options, but that should never be the starting place. Last resort methods imply that other less desperate things be tried first.

FWIW, he's not trying to unscrew a part that broke from torque. He's unscrewing the crank bolt, and it will be difficult but not impossible with a wide blade screwdriver, aided with a wrench for torque. It has nothing to do with the part he broke, and his fortunes depend on how tight he tightened it when he installed the crank. The ideal tool for the job is the hammer impact wrench because it turns the blade while driving it in. (BTW, if he has an auto mechanic friend, many have these for similar jobs)

BTW- while the Dremel is handy, the OP could extend the slot into the ring of the extractor tool, making a small notch. If he threaded it in by hand only, he might be able to set a small punch in the groove and gently tap it around. Problem is he can't use much force because that'll drive the ring into the crank making it tighter. It's worth a go if done carefully.