Breed a ton of Scatterbugs, like 8+ boxes. Then, deposit them in the GTS asking for a pokemon you can't get in game by yourself.
Meanwhile, catch all the pokemon you can, including the ones in the friend safaris. Catch extra's to breed/evolve them to get all the forms.
That should get you everything save the legendaries. For those, you'll either have to transfer copies from previous games, if you have them, or request help from a clonner or genner from somewhere.
Doing that got me my first living dex (also marked the first time I've bothered to complete the pokedex - and I've been playing since the beginning) in a little over a month, not including time spent beating the game.