>>24857271Just deposit a popular pokémon in the GTS and wait for a few hours or a day. I got many legendaries that way.
My main tactic was trading Gallades, I just had to caught Kirlias in a Safari Zone (Y), and play the specific Super Training that could drop Dawn Stones. You can see here:
http://bulbapedia.bulbagarden.net/wiki/Super_TrainingIn 3~7 tries generally I could get a stone, but if I really wanted to get a certain legendary/trade evolution/rare monster, I would get 2 stones, the first one I would use to evolve Kirlia, and the second one I would make the Gallade hold, so this would make the trade be even more faster.
You just need to learn how GTS my friend, it's the best way to fill the dex nowdays without having all the games.