If there's aluminium corrosion in the threads you may never get it off, at least not without bending something that should be straight. Even if you manage to grip the locking ring securely, you still have to somehow apply a torque to the barrel or shutter, and that can munge the shutter body.
If locked in place by corrosion, cutting the ring off, cleaning the threads and finding a new ring may be the safest option. Cut one or more radial slots in the ring and prise it out and away from the shutter/barrel thread),
I have applied the largest, most steady, and most stable forces to rings by locking two screwdrivers upright in a bench vise. This lets you use two hands to hold the shutter/barrel and turn it.
Hammer tools are great for breaking a locked screw or bolt, but I don't know of any that will apply an even impulse around a retaining ring, or even on two or three slots around the rim.
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