How long has it been on there? What wheel and what cassette?
Do you remember if the threads were greased? If they might not be, squirt some penetrating oil (or CRC 5-56 or WD40) and let it soak in. Repeat. Then a hairdryer to heat things up and expand them a little.
Do you have an inflated tyre mounted on the rim? This gives much better purchase than a bare rim.
I have had a few tough to release cassette lock rings. Always found the best approach was to apply even, heavy force to them slowly. Hitting them, or trying to shock them into moving is not going to work.