They can be total bastads - my last one took over a week...tried freeze and unlock spray...tried vice, tried bolt through seat post...tried heat. None of them worked. Padsaw and cutting down the length of the tube and twisting it in on itself was what eventually got the effer out but even that wasn't plain sailing cos its murder trying to get an even cut and working out how far through you are. I ended up trying to twist it in on itself too early and it didn't work right so I was in with a 14mm titanium drill bit to clear the sh*t out of the way to let the saw blade back in again....several times!
I know it'll be doing your nut in but sawing will work...eventually!!
Good luck...perseverance my man!