This is one of my favorite problem cases.

Two ideas:

-get a gear/wheel puller. Depending how the arms of the puller fit between the arms of the crank, this one can work beautifully. The pullers of this size are a bit pricey, so maybe you can borrow one from a friend who does lots of car repair work. BTW, do NOT pull on the chainring; pull on the spider itself. Not all pullers can get a grip on all spiders however.

-loosen the spindle bolt a turn or so. Then go for a short ride. Maybe a couple of miles. This might loosen the arm enough. This one might distort the hole in your crankarm, but then it is toast anyway with stripped threads. Don't take the bolt all the way out, btw, since the arm might just fall off.

Still no good? Then beat the snot out of the arm with a heavy hammer. Try to get some penetrating oil down in there as well as you can. Beat it some more. Go inside, have a fight with your loved ones. Come back out and beat it some more.