Tom Hoskinson recommends 99% isopropyl alcohol applied only to the threads, and often enough to keep them wet until it soaks through enough for some pressure to release the threads. There are specialty metal and synthetic material friction wrenches that you could find and use from a supplier of camera repair tools, but that's probably a bit over budget unless you're going to do this repeatedly.

I soaked one of my folder lenses in lighter fluid, not recommended by more experienced people than I, as it can deteriorate the coatings and cause cemented elements to release. I got lucky with mine, but it still took over 2 days of soaking to get the green cement, I mean grease, to release, and it took some of the black finish off the lens cells.

Patience is a virtue. I've heard from people who have used too much force and cracked lenses.

I haven't tried Liquid Wrench, but it sounds promising.