try holding the bolt with a hex wrench and use a punch or screwdriver the loosen the nut (female portion). you should be able to give both the slots a couple of good whacks before it becomes so destroyed as to prevent further attempts. then i'd just punt and drill it out, i guess.
i won't be surprised if somebody has a better suggestion ...
you might even attempt to file down the too high middle part to give your removal tool a better purchase. it's pretty hard stuff though.