An easy out is the first answer, but if its a real small thin head, it may just eat the head without moving the screw.
Depending on the thickness of the ring cap, you may be able to drill out the head of the damaged screw, remove the ring cap and scope. Then grap the remaining piece sticking out with a pair of vice grips and back it out. It shouldn't be under tension when you remove the cap so it should just back right out.
Did you use Red or blue thread locker? Then you will need to apply heat to liquify the thread locker before you try to twist it or you will just snap it off in the ring.
The other question is the cost of a replacement ring... is it cheaper to just drill out the scre head to get the ring open and replace it with a new ring vs the cost of a screw extractor and your time to remove the old screw.