Fuel filter removel

Personally, I'd keep trying to get the original nut loose, or take the line still attached to the filter completely out and buy a new one. Just do your best not to twist the line or kink it, because one that happens, it's time to buy a new one anyways.
I know flaring and cutting tubing like this because I allready have the equipment for it, and have done it (on occation) with my own vehicles for brake lines, but NEVER the fuel line.

Keep trying to get it off though, and it's ok if you totally destroy the old filter in the process. Worst case, you'll damage the line still connected and have to replace it anyways. Vise grips come in handy in a case like this too. It won't look pretty when your done, but it'll work in the end. I'd rather hear you got it off, or bought a new piece of line to replace the old one, than go through getting the flaring tool and pipe cutter, and have it possibly leak fuel.

What about some rust remover?

ANYTHING that seems to work for you to help loosen it. WD40 ECT. For starters. Just check and ask at a local parts store really, because there are better availiable. But:

worst case, 2 pair of vise-gipis.1 on the filter, and the other on the nut, as tightly as you can (to cause less damage to the nut in this case). Clamp down on them both and go for it. Iv'e had to do it with a brake line before (similer). Not professional, but it can get the job done.

Keep in mind. If the fuel line has a lot of rust, and seems like it has some deep 'pitting'. As in spots that look like are deep within the tube with rust. Then just say 'forget cost'. Replace it. Maby about USA $30 at an average if it's the tube going to the engine. If it's to the tank, it'll be less.

Let us know how it comes out though, and I'll check every afternoon to see how the information I gave helps, and any other help I can give on this.