Personally, considering the cost of a replacement crank and/or chainwheel, I'd probably take the bicycle to a trusted local bike store and pay them to fix my mistake. If you make it worse, that's false economy.
You could slot it with a Dremel, but before I did that I'd try a Torx bit first. Sometimes it is possible to force a Torx bit into a stripped Allen/hex-headed screw, and if the bit is short enough you might be able to force it in using a clamp of appropriate size. Unless you protect the area around the bolt and have some solid Dremel skilz, I'd say the chance of marring the chainwheel and/or crank is pretty high if you go the Dremel route.
Heating the bolt with a butane torch lighter or propane torch could help, but I would not put the whole crank into the oven. You might try penetrating oil, followed by some heat, followed by some more penetrating oil, etc. Be careful not to overdo it; I would start slowly, and go slowly. I should note, I'm not a metallurgist, but I would think that too much heat is as bad as not enough.
Too, I've never tried this, but it might give you some ideas.