Long nosed pliers.
I tried a lens spanner wrench and no luck. Maybe I need to eat lunch first. LOL
Put some effort into it! 
Have some spinach.
Oh great, now we’ve got Nevs attention
LOL
Lucky you.
hahahaha. I’m serious I have the head in a vise. Not budging. About to give up.
Use a big hammer.
I’m ready to use a blowtorch
I’ve never seen a glued retaining ring in these but could be, maybe a little heat?,
Loosen the front bezel a half turn too.
Yeah. The one I have definitely had threadlocker between the head and the battery tube, so wondering if some of that might have seeped towards the retaining ring. I’ve tried clamping down hard on the outside of the body to see if it might just be frozen in place but again, no luck!
Mine had black thermal/potting compound leaked down almost to the battery tube,
I’m going to stop messing with it for now. Hands are completely raw due to the super aggressive knurling.
Use some oil of Olay .
I remember when it used to be Ulay. Marathons, Opal Fruits, Treats.
I have good news and bad news:
good news: I got the retaining ring off
bad news: I sheared the pcb off the bottom of the pill 
Use the bin.
You’re supposed to say “make it into a triple”
Make it into a triple.