If I recall, I had a similar problem with my Sea Dragon light. The problem for me seemed to be that I just couldn't get a good enough grip to twist it off. What ended up working for me was one of the round rubber jar lid grips. It still wasn't easy but, it gave me a good enough grip to twist it off without damaging the end cap.
Once you get it off, inspect the O-rings to make sure they aren't damaged. If they're okay, put a small amount of silicon lubricant on the O-rings. Just enough to make them look wet (no gobs). It could be that the person who assembled it at the factory neglected to put silicon on the O-rings. Lack of the silicon lubricant will cause the O-rings to stick to the cap making it difficult to remove. If the O-rings are damaged, replace them. The light should have come from the manufacturer with a spare set of O-rings.
