Is this just a billboard quad with a bloom shader?
Although I think this is a very subjective question, the ways I can think of creating them are:
As you said, billboarding a plasma sprite and using a bloom shader
Creating a particle system and using a bloom shader (will look better than above, but takes more power)
Both, which by looking at the picture, I'm inclined to believe is the method used in that game. It uses a main explosion sprite and then some particles with an additive bloom shader for sparks.