Reggie might be onto something here.
I used to have to do something similar with a shimano compatible Hope hub that I could never get the bloody end cap off of.
I used to partially undo the cassette lock ring and then lever the whole lot off by wrapping a chisel in tape and cloth and using levering the cassette spider leg against a spoke base. Not ideal, mind.
I’m pretty sure it caused some damage to the spoke ends and the biggest sprocket used to flex a lot sometimes.
But it sounds like you’re on a hiding to nothing and need something that puts a lot of outward leverage on the cassette whilst unscrewing it. You may need a helper as I think we have just exceeded two hands worth of tool use!