My guild taking down the Twin Val'kyr for the first time too bad we missed the achievement timer by just 1 second D:
In this fight you fight a light and a dark val'kyr, the raid can use essence of light or dark to do more dmg to the oposite of what they use. after that you can collect any sort of light/dark energy which is the same as the one your using to power yourself up for even more dmg.
Once in a while one of them pops up a shield and begins to cast a heal, break the shield quickly and then interupt the heal. Thats pretty much all there is to it.