I always use a flat sided socket on them. One of the bolts needs a shallow socket iirc
With a rusted or rounded bolt you can hammer on a smaller socket (imperial sockets can often work out a good size for this) and you can also try heating it but let it cool a bit before turning.
I would not try drilling it out as that is more likely to cause further issues. If a very stubborn bolt the best answer is to weld on a nut to the head putting in as much heat as you can
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