Atra du evarinya ono varda.
Hmm..weird indeed. I don't really know how to solve your problem. :/ They could be hiding in other boxes..just a thought.
Yea, but things have changed.. Yea you could go with Gyarados and its Thrash. Only that Gyarados isn't a dragon-type; it's a water & flying type. It just looks like a dragon. I made that assumption before I found out. ><
Kingdra (her strongest) is level 41. So you should perhaps raise them around level 35 - 45 to be on the safe side.
Status-inducing moves are useful too, especially moves that cause a burn such as Will-o-wisp (they reduce the opponent's attack to half) and paralysis such as Thunderwave (they half the opponent's speed). Only if you have them, that is.
For Kingdra, just try to hit it as hard as you can with your strongest moves that are normal towards it. That's kind of what I did.