Provided it's spinning freely in the bush in the swingarm, you should be able to force a blade (screwdriver, chisel, etc) between the bolt head and the clevis on the shock, if you then
carefully apply a levering force behind the bolt head whilst
carefully turning it, that should be enough to persuade it to twist out.
I say carefully because it's all to easy to twist the lever out from under the head and either, impale yourself or, worse, damage the bike
Until it's out there is no way of knowing which thread is actually stripped, but the chances are that it's the bolt i'm afraid.