I soaked it in PB Blaster, then sawed off the steel pipe where it exited the nut; then I was able to get a socket on it. After bracing my 3/4 inch backup wrench, I was able to use both hands on the breaker bar and socket to stabilize it and keep as much stress as possible off the evaporator. The nut turned and I got it off, apparently without damage to the evaporator.
Thanks everyone for advice and moral support!