Not much. Since you already have Sun, play it through. If you really like it though, I would highly recommend getting US or UM. You don't need to play them to get the feel for the generation, they just added a few things.
To bounce back to your original question, I am not sure if you watched a video on this or not, but to get all of the non legendary Pokemon that are still hard to get, using a starter from gen 4 (the only generation without a remake) will get you places. I used a Piplup to get almost my entire live dex xD. Just stick em up there and check back in an hour or so, its all you need. For legendaries, the trade corner of this forum can help you out, and i can PM you a list of some spares i got if you want them.
Also, you may not be getting much action as X/Y isn't as popular with Gen 7 out. You will probably find better luck using the GTS in Gen 7.
Good luck!