Try this- Ride around, pushing really hard on the pedals, trying to get that cog as tight on there as you can, but DO not resist the pedals, i.e no skids/skips. Use your front brake to stop, if you have one.
After a little while (30 mins maybe?), yor cog will be really tight, then tighten the lockring again, so its really tight. Make sure that you do this on a side street or something so you don't kill your self.
The proper tools, a lockring wrench, and possibly even a chain whip will help here.

Loctite should be a last resort, but use blue strength, so it is not permanent. No JB weld/Red loctite.