Worst case scenario:
Use a drill with a small drill bit (maybe 1/8") to drill some holes through the retainer in order to weaken it in a few locations. Use gentle pressure so you'll feel when you bottom out against the steel of the hub. Then take a chisel and hammer, and catch the edge of the retainer, and then hammer towards the axle, parallel to the spokes. This will break the retainer and push it "in", that is, away from the threads it's engaged with. Once you get it moving you'll be able to pop the remnants out.
I've done a couple front retainers this way when they were too chewed up to remove normally.
mystic_1