When I was a youngster, my Dad's response when I said "I can't" was "Can't never accomplished anything."

As a previous poster said, liquid wrench and some good smacks with a good sized hammer. If it is in a location where you can do it, I would think that a six point box end wrench would let you get a little rougher with it. A cheater pipe over the end of the wrench if necessary. It's hard to know what will work without seeing its position.

When I run out of enough curse words, I try making up new ones.

Go at it with determination, a cheater pipe and you will get it!

Best of luck,