since you're changing the struts and keeping the springs (I'm assuming, since your topic says changing strut) -- just drill the motherfokker out.
My friend had the same problem when changing the suspension on his FC; the struts came off the car easily, but when he tried to remove the upper mount, the nut just turned the piston.... we were sooooo frustrated trying to get that **** off, tried everything.... breaker bars and sockets, vice grip and socket with breaker bar... everything. In the end we went out and bought some uber-hard drill bits for drilling metal, and tore that shiz apart. :P it wasn't fun(ny) until we got that **** off and the coilovers (no pillow mount) back on his car.
