[mark0™]

Yeah, you are supposed to kill it with the opposite element to increas chances of dropping Rare items. And to increase your chances, use Djinni on the character casting the final fire attack which have increases on Luck.

I've tried that routine before Nelo, many many mnay times, but never got it

But I think we only get to play as Issac's party at the beginning of GS2, then we play as Felix and Co. on the floating island... So its not essentialy going to make the game tonnes easier.