Yesterday I bought a new cassette for the road bike but for the life of me I could not get the old one off. The lockring & outer gears came off but then the rest of the cassette was stuck VERY firmly to the freehub.
Merlin (who I bought the cassette from) are just round the corner so I popped in and the kind mechanic sorted it for me for free.
The technique for future reference is two chain whips one in either direction, one on the smallest stuck gear - give them a jiggle and hey presto. repeat for next stuck gear.
The cause - a lightwieght free hub body. - the cogs have pressed in to it and left little indentations - cauising cassette gears to get stuck.
grease would not help