If it’s left hand threads, you’ve now got it super tight. Double check the start of the threads if possible and try to determine which direction the threads run before you end up tearing something up….the light, your tools or yourSelf! 
Thanks for the suggestions folks. Nah, I tried it backwards and forwards and even put it in a padded vise to keep the tailcap from rotating with no luck. This thing is locked in there like a bank vault! I’ve never had one I couldn’t break but this is it .
Well, if you really really want in, you could try drilling the indents out and making them full holes, then make a tool to get a much better grip…say an aluminum bar with 2 drill bits spaced to match the holes.
Push come to shove, you could always split it with a dremel tool. (the retaining ring, I mean)
Actually Dale, I thought of that but the holes are deep enough that I can get a really solid purchase with my circlip pliers which have very strong tips and still can’t budge the dang thing. I don’t think I want to split the the threaded retaining disc.
It’s weird that Nitcore would include an extra rubber switch boot with no way to replace the original, at least on my example. I guess I’ll just have to live with the black boot. 
Oh yeah, if anyone has any luck getting theirs out, please let me know!
No luck. It's glued in very hard.