WD-40 - let it soak in and sit for a few days, maybe even a weak. And just work the parts apart with torque. try and loosen after a day or two of soaking with the WD-40, then soak it with more, let site for another day or two, try again.
I had a Crystal Reveal Prodigal Son that was sent to me for repair that was all glued together and was a giant PITA to get apart, took almost a week for the internal chamber/coupler to loosen up so I could even get into it.