User blog comment:ImperiexSeed/ARCHANGEL or LEVIATHAN!/@comment-149.210.105.71-20130514175442/@comment-3481504-20130514175823

No, no extent of Castiel's powers were diminished while in Purgatory. Because, as a Seraph, he retains all of his Angelic power. Of course the Devil could kill the Leviathans.