The drain pipe connection is frozen. The nut under the sink is corroded and very thin. It is difficult to grab with any kind of tools I have. And when I barely grab it, I can not exert enough force to loosen the connection. I sprayed plenty of liquid wrench. And when I manage to turn it a bit, the entire joint including the pipe and the nut are turning. Can somebody help?

Easiest thing to do is cut the nut with a hack saw, remove it and replace with another metal nut or convert to PVC. Common problem with die cast metal.

Thanks.