Are you spinning it counterclockwise to loosen it? Sheldon Brown's site says you need a special tool to fit the splines, but you have done it before with two chain whips?
-type cassettes (everything made since the late 1980s) us a threaded lockring to hold the sprockets onto the splines
of the Freehub body. There is a special splined tool that fits the notched hole in the lockring. Some lockring tools have a long handle, others, as shown below, have a hexagonal fitting like a nut, which can either be turned with a large wrench or may be placed in a vise. The lockring has a normal right-hand thread, turn clockwise to tighten it. When you want to remove the lockring, you need to turn the lockring counterclockwise, but the cassette will freewheel when you do that, so you need a chain whip to hold the cassette from turning.
You can see the 12 spline notches inside the lockring. Lockring tool and chain whip in place, ready to unscrew the lockring.