Well, sheesh, that's on there bad. Maybe someone used locktight on it? I guess, then, I agree with Streido, you might just have to cut it off. Yeah a dremel with one of those thin carbon cut-off wheels cutting as much as possible -- then a cold chisel to crack it the rest of the way.
That can get a little messy since you might have to dress up the crank threads afterward in which case gotta have a die and so on to do so. Not to mention having to order new nut and washer.
By the way, I found a hairline crack in my washer once. Maybe I had been tightening to much previously. Anyway I used it till a new one came in.
Situations like this make me wonder how an experienced shop mechanic would proceed? ?