Just as a basic reference, you can use heat, cold, solvent, impact. If there is anything at all sticking out you can try to grab on to it. Sears sells a variety of screw extractor sets. One important concept is that they all have reverse twist drills (they cut in reverse). That way if the drill digs in it will have a tendency to loosen the bolt not tighten it more. The hardness of the bolt sounds like a problem but maybe a fresh bit might work better. It shouldn't be so hard that you can't drill into it. I am not familiar with that specific bolt. Maybe someone else has a better idea. You could try searching. I have seen similar threads before about broken off bolts.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra