Not very hard actually. I don't think he did it intentionally, so I don't know exactly what happened there (have a few ideas though), but to do it consistently all you have to do is Reverse Rainstorm (please Capcom rename it to Rising Storm), Angel mode, double jump and prop. Wait a bit after you input the reverse rainstorm, but do the rest of the inputs (L2, X , L2 + O) kinda fast. You'll now you got it right when Dante has the Osiris on his hands, instead of his back.