Use needlenose pliers opened to fit the 2 dimples in the retaining ring shown in your photo. Unscrew it and everything should come out of the head end of the light.
Any luck? It’s possible that the threads were glued to maintain the alighnmet of the port in which case you might need to apply hot/cold cycles and break the adhesion. Usually it’s the tail cap switch retainer that has left hand thread and pills are right hand thread.
I’ve done that exact thing when working on car engines trying to get screws and bolts in place. Usually it caused the engine to rethink things and things went well afterwards. Felt like doing that with small gun parts too, especially revolvers with their myriad of small parts and springs, but always thought better of it before doing so