My personal superstition is that the quicker you kill the enemies that drop stuff, the more likely it is you'll get it. I also tried varying my team just in case... still nothing. Even on my two attempts on Cornered G, I only got a single Demon Crystal each, which is the biggest "screw you" I can think of for mission drops... giving me something useful in such a worthless quantity.

I swear, this game tries to frustrate me any way it can.