I don't know if this is the most efficient method. This is the method I use, until someone tells me a better way. You don't need to be anywhere close to my level to get these results, but then it doesn't take too much effort to reach my level in the first place.
I typically get 700-1000 cores for a cost of 168. I don't believe premium is necessary. But I love this game, and chose to support it.
