Before you do anything else, open the aperture blades all the way and engage the preview lever
to open the shutter blades. This avoids damaging them if something slips.

Do NOT use screwdrivers or a caliper on the retaining ring. Only use a proper spanner.

Do NOT use a strap wrench. It can damage the shutter.

Grasp the shutter firmly with your right hand and turn it counter-clockwise.
It sounds as though someone has used some thread locking agent.

- Leigh