This question is very funny to me, look at B to see why. :) Okay, in order to get Legendaries from other games that aren't in X/Y without trading them to GTS you can either:
A: Make a Shout-Out e.g (looking for) LF: Entei
Chances are that some wonderful person will respond by sending you a trade request and from there the two of you can negotiate.
B: Take advantage of PokemonDB's Chat Room! Chat is usually filled with people who want to trade just like you. In Chat you can set up a trade between you and someone and actually tell the person " I'm offering Shiny Deoxys for a Groundon." Although it's not certain kind people here are willing to trade with you for an equal gain.
So Hopefully I've Helped