Hardened bolts

What I've had luck with in those situations involves heat and an oversized bit to drive in there. Once you drill a decent-sized hole through it, get it red hot so it's soft, and drive something oversized into the hole while it's nice and soft. For smaller fasteners a Torx bit a little larger than the hole works well because the torx bit cuts its own 'teeth' into it and gets lots of grip. For large fasteners a large hex key, or even a socket extension ground flat-ish works. Something that will cut its own grooves/teeth. Then try spinning it out while red hot. If that doesn't work, let it air cool with your torx bit/whatever in there, then heat the surrounding area red hot (properly red hot) and try spinning out. You'd be surprised at how easy they can spin out when it's nice and toasty red, vs. how seized or stiff they still are when it's only a dull/colder red.

Whatever you do however, don't try twisting too hard and break off whatever tool you've driven in there. Then you'll have an uber-hard piece of tool steel in there that will be impossible to drill through.