are you sure this is a cassette hub? this looks like a lockring on a freewheel and isn't designed to be removed. rather, you need the proper freewheel remover to engage the splines visible just above the axle. also, you will not need a chain whip in this case.

@Sixty Fiver - it looks like the freewheel in question is one of those old ones from the late 70's that had a steel barrier beyond the smallest cog, so the chain can't slip off and stick between the dropout and the cog.