As for Dialga, I never really had trouble with it because I always had a type advantage against it (I was a Charmander in Darkness and a Riolu in Sky). I pretty much made short work of it with about 8 Flamethrowers/Force Palms. And after I evolved my starter, I went back and beat it in half that many moves. Darkrai, though... I don't remember how easily I beat Darkrai in Darkness (it must not have gone too badly since I don't remember it that well), I went back and beat Sky, and even with the type advantage, he gave me a ton of trouble. He would either put me to sleep the entire battle and either I, my partner, or Cresselia would die before I wake up, or he would spam Double Team to the point where none of my attacks could hit him. Even when I was able to save 6 Reviver Seeds and enough Pure Seeds to essentially warp my way through Deep Dark Crater, it still took me a few tries to beat him.