Cassette lockring too tight

I need to replace a cassette and have been trying without success to loosen a lockring on a standard Shimano HG50 cassette to the point that I've now even bent the chain whip.....It's something I've done many times successfully for customers on various wheels when removing cassettes over the last thirty years and I do have the proper Shimano tools for the job but this wheel has me beat, I've tried penetrating oil, using a vice plus using an extension bar on the spanner for the lockring tool but the end result now is a bent chain whip and a broken vice for my efforts, can anyone offer any advice? fingers crossed, Griff

Do you have access to a vice?

I did but managed to break that as well, I've now had to order a new one....believe me this lockring feels like it's been welded, probably have to try a LBS

Have had this in the past, angle ground gears 2,3 and 4 flatsides opposite each other like the flat sides on a nut, pipe wrench on the flat sides and cog tool on a 2 foot breaker bar.

Not failed to get one off yet. But this is a last resort.

Tried heat? Someone may have put thread lock on there. Although I doubt it would be that strong. Even so, heat can get anything off

1/2" rattle gun. I'd almost guarantee it'll do the trick.

Failing that, an angle grinder will definitely get you there, with a bit of care and patience.

Thanks everyone for the advice......finally managed with a heat gun and impacter!

I guess it goes: cassette tool > large socket > impact driver?
Smart...

Never had to try it, but impact drivers are a game-changer. A 1/4" one is definitely worth a shot on a lot of places on a bike you might have a seized fastener.

Goes hand in hand with hammering a torx bit into a rounded allen head.

Yeah, I slightly rounded a bolt trying to undo something. Came out easily with the impact driver.
Just impact driver everything is my new motto