i always leave my tire on my wheel when removing a cog.
put chainwhip around the cog, then, on a soft surface like a carpeted floor,
gently bounce the wheel while torquing the chainwhip,
make sure that you are going in the right direction though (it will be opposite to the lockring).
after a few bounces and torques, it should break free, but if not,
then try the other suggestions on here...