the trouble with distorted and notched freehub splines is that the notches are not square, they ramp up and the torque created by riding tightens the cassette onto the freehub body. it can be very difficult to get off. the displaced freehub body material can make it even more difficult that one would think. there are probably 60 to 80 notches on a notched freehub.
the hard part is that there is precious little to hold on to while one tries to dislodge the cassette, that doesn't also turn with the cassette. seeing as how the cassette must be turned slightly counter-clockwise to dislodge it, and the freehub is designed to freely turn counter-clockwise too.
the problem and it's solution are similar to the one presented by a freewheel threaded on to a hub, sans rim, and that needs to be removed. ...nothing to hold on to. ... and elusive.
sometimes, two chainwhips (or one by simply shifting on to the small cog and immobilizing the crank) can help if the cassette has a separate small cog. by attaching one to the smallest cog and one to one of the middle cogs, and turning them in opposite directions, with some effort, it's possible to dislodge the multi-cog carrier counter-clockwise.
Last edited by hueyhoolihan; 10-23-15 at 07:36 AM.