>Completing the Pokedex in X
Focus primarily on catching the ones that are at your disposal (that means those that you can easily catch on the game itself or transferring from other games). That's almost 25% to 50% of the entire pokedex, if not more.
Then focus on giving away baby starters (completely untouched) in exchange for normal pokemon not as easily acquired. The rarer they are, the more chances of a trade occurring.
Once you've got that out of the bag, all that's left is pseudo-legendary pokemon (which might not be as easy to trade with baby starters) and actual legendaries. There's a trick that's called the... Never mind, I forgot its name in English but simply put, there are people who give away their legendaries for pseudo-legendaries (don't ask me why, it's their own fault).
By pseudo legendaries, I mean shit like Dragonite, the good old Goo dragon, that kind of stuff. You wouldn't imagine the kind of legendaries I'd get just with a Goo dragon. Of course, it doesn't always work, but as soon as you get a legendary, take advantage of it to trade for OTHER legendaries. Of course, the rarer it is, better the odds of getting a wanted legendary.
AND THEN THERE'S THOSE LITTLE FUCKERS THE EVENT ONES
You MIGHT get lucky if you manage to get them from friends, but all in all, they're actually not that fucking necessary.
