I don't know about Sephiroth, but I used it against the two headed dragon in Gaea's Cliff. It combined with Comet can smack pretty hard. Other than that, I found it pretty useless, because you get Neo Bahuamut and Bahamut Zero fairly soon afterwords. Plus, I don't remember where the Huge Materia fits into all this, but Hades is one mean summon with added effect linked to it in your weapon. It's really good in battle square.