If you're a good Ninja you already have Fuuton up though so that you don't have to waste a Ninjutsu when AoEing just to apply it. So Katon -> Kassatsu -> Doton is the correct answer, provided the mobs are going to live for the duration of the Doton. Otherwise just do Katon -> Kassatsu -> Katon while popping Death Blossoms.
