Suggestions in order of DEcreasing desirability:

A)Mill it out on a large rigid end mill, may need a carbide end mill cutter.

B)Grind it out with a(your) dentist's carbide insert(s) held in your hand grinder, drill press or mill. He/she should be willing to give you plenty of old used ones or even sell you some new ones. They're cheap.

C)Use a center drill or small grinding point to form a center dimple and then use a left-hand bit or E-Z-E Out to back it out.

D)Use a center drill or small grinding point to form a center dimple and then use a right-hand bit (may need carbide) to drill all the way through the screw.

So, doyathink I've hadta remove a few screws? (grin)
Regards and good luck, Joe

You can lead a man to logic but you can't make him think. NRA Life since 1976. God bless America!