To get more of an alpha transparent render you can render it normally but with the background color a bright shade of some color you aren't using on the rendered figure ("green screen" it, render with a bright green for the background). Open it in PS and use a Color Range selection (200 fuzzy) to select and delete the green (any remaining green is remedied by the color balance), then follow the rest of your tutorial.
My only critique would be that the brushwork in that DA piece is very blatant to my eye (areas need to be layered more in transparency), and it seems more like brushwork for the sake of brushwork. Continuing with my policy of "if it's an abstract, then you're probably an idiot (

)," I would say that if the big spiky thing isn't going to have a theme, then at least give the brushwork a theme. What you have it a big spiky thing with some big glowy stuff. You could have made the brushwork glow more circular in addition to some more mouse work to create a much more convincing and spectacular explosion effect, or you could have at least given the glow some sort of source... or purpose.