This was always my problem with Superman
It seemed to be entirely up to the whim of the writers as to what did and didn't affect Superman. Good idea though, surely at least he could hurt himself.

That said, Superman did eventually get the shit kicked out of him by Doomsday so maybe he's only invulnerable to earthly stuff like bullets an' that. In which case Sephiroth's fuck-off bastard sword might be reasonably effective
(Darth Foxtrot A one-man army dedicated to making fetch happen, Wed 29 Sep 2010, 10:20, Reply)