I think pretty much any villain could defeat it.

Sephiroth: Did anyone forgot he summoned one major Meteor, if that were to hit 'Sin's hard Shell' even he would be defeated, considering the holy materia is not there to protect it.

Ultimecia: She tries to make time-compression, and only she can exist in such a world... so technically, sin wouldn't exist anymore and such defeated.

Vayne: Well, he's got Bahamut Super Sized Ship. Yeah.