See if you can get a long pole to fit over the handle of a chainwhip for heaps of extra leverage and have someone hold the wheel secure while you yank down on the sucker. Soak the the cog in WD40 or similar overnight beforehand too. I had a bast4rd of a cog a while back that I bent my chainwhip on and thought would never let go too, but this technique ended up working.

And yeah, don't forget to grease the thread on your cog next time either ;-)