I have to agree that alternating seems to be the way to go. I found two of those cores while killing about 30 elementals as a level 46-47 warrior, then I read about the alternating thing, and doing that I got four cores from five kills so far …

I'd also advise on picking up everyhing the elementals drop, even stuff like elemental water and enchanted water that are not use to many classes. It might just halp :)