I've never been able to do it right but I think the hardest electric guitar technique I can think of is playing one or more notes, bending one up whillst pressing down the whammy bar so that that note stays the same while the others fall in pitch.
A similar thing could be done by prebending and releasing one note whillst bending another up. Horribly difficult I think.
Roy Buchanan used to do some crazy stuff with bending two different notes by different degrees at the same time, that's manically hard.
Violining pinch harmonics isn't easy either but I can do it now I've put in hours of practice.