Sephiroth was unfortunate enough to exist (in reality, it’s an scp reference and the scp killed 682 who is supposed to be unkillable, good thing it’s a joke scp)
Win doesn’t mean a clean kill, I think that he would have the power to make 682 afraid of him and he probably does have the power to kill 682 if he worked for it. It’s an even match as long as the black materia doesn’t come into play.