You have the weaknesses wrong there I'm pretty sure. Earth element is weak to lightning. Lightning is weak to wind. So in fact if Kakashi used earth element he would be at a disadvantage to Darui. In an elemental battle it comes out pretty even between the two. Kakashi has 3 elements (4 in the anime) and Darui has two but he also has a KG so that could count as a third.
Darui- Water, Lightning, Storm Release
Kakashi- Water, Lightning, Earth, Fire (anime only)
And to the person who said Kakashi could copy black lightning, yes that is true if there aren't special circumstances that allow a person to use it.
