The key word here is 'exclusives'
[Also, feel free to check out my full Pokemon X and Y tips guide.]
Over the past week, I've been slowly working my way up the Pokemon X and Y trading chain. I started off with a minimal amount of knowledge with the GTS, encountering error after error until I abandoned it, and now I'm almost 100% done with viewing the Kalos Pokedex (which, broken into three parts, totals 454 Pokemon).
It all culminated in a trade I made yesterday, in which I gave away a Kangaskhan for a Xerneas. Read on to learn how I did it, and what else I learned about trading in the past week.
Learning the Global Trade Station (GTS):
First things first -- get accustomed to the GTS system, which is found in the online menu of Pokemon X and Y. The GTS allows you to do two things: search for potential trades, and offer one Pokemon at a time to be deposited and traded. The former is what you'll most likely spend the most time on as you learn what trades for what, but the latter is where you'll actually get the bulk of your trades from, with some patience.
Keep in mind that as long as you learn what is and isn't a fair trade (which I'll explain below), you'll be able to exchange Pokemon through the deposit portion of the GTS in seconds flat. The Wonder Trade system (you can put a Pokemon up for trade to earn a random creature) can yield some great Pokemon, but the time spent with that could be better spent with the GTS. As a side note, if you're searching for Pokemon and turn on the filter "only show Pokemon that I have to trade," you will most likely encounter a glitch where the GTS shuts down, forcing you to restart your 3DS. Don't use this filter for a more stable experience.
Once you have grown accustomed to the GTS' interface, it's time to actually catch some creatures and start trading. Here are the exclusives for Pokemon X and Y, without including Mega Evolutions.
Print off this list, or start memorizing it.
You might have noticed that a lot of these are actually old Pokemon from previous games. Right now, this doesn't matter, as the transfer app isn't scheduled to hit the eShop until December 27th -- so as of this moment, all of these are technically exclusive to the Kalos region. Use this time wisely to max out your 'Dex while their values are up.
For reference, the new National Pokedex (which you earn after completing the game) now goes up to 718.
Learn the Pokedex:
If you're looking to amass more Pokedex entries, master the art of equivalence. In other words, learn what exclusives match up with the other game, and you'll end up having trades go through in 30 seconds or less. Take Sawk and Throh -- both are found in the exact same area (the grass right after Geosenge Town and before the cave), but only in X and Y respectively. As a heads-up trade, these exclusives will most likely be exchanged instantly if you deposit them through the GTS.
A lot of these actually match up perfectly, and can be found in the same area -- such as Skrelp and Clauncher, who can both be caught using a Good [fishing] Rod on the shores of Ambrette Town. Exclusives don't necessarily need to be a 1:1 trade -- so if you find a spot with any of the above Pokemon, catch five or six of them and keep them in a box. What I usually do is create an extra box for duplicate Pokemon specifically for trading to avoid confusion.
If you're looking for all six starters (Froakie, Chespin, Fennekin, Charmander, Bulbasaur, Squirtle), all you need is your first "in," which can be a rare Pokemon. Rares are creatures that have a 5% or below encounter rate, and can be traded at a high value for pretty much anything. Kangaskhan and Abra are examples of rares, so if you find them, capture them at all costs.
Really though, considering most people are breeding Dittos with starters to trade for exclusives, any exclusive may be enough to get a starter. If you want to Ditto-breed yourself, you can trade for one with a rare, or just catch them yourself near the end of the game, in the Pokemon Village past Snowbelle town.
Finding the right Pokemon:
To encounter certain types of Pokemon, you need to know that each type of grass (green, yellow, red) features a different percentage encounter rate. For instance, right above Cyllage City you'll find a path with stone pillars, and in this section you'll find green and yellow grass. Eevees are a "common" occurrence in yellow grass, but "rare" in green patches.
You'll learn these rates over time -- so if you find that you're catching the same exact Pokemon over and over in a certain grass type, go to another color. Also, breaking stones with Rock Smash, and the Old, Good, and Super Rods all have different Pokemon encounters, as well as Surfing on the water's surface. Use all these methods until you've exhausted all of the types in a region.
Learn to catch correctly:
Do you need help actually catching these elusive Pokemon? Make sure you buy a decent amount of Dusk and Quick Balls, as well as the best normal-type Ball you can afford (Great, Ultra). My rule of thumb is to always have 50 of the best Pokeball available at minimum, so I never miss out on a rare catch. But no matter what Pokemon you encounter, throwing a Quick Ball at the start is my go-to method. This works two-fold in that it's one of the best ways to bag creatures that may run away immediately, and it also has a high capture rate for everything else. If it fails, resort to Dusk Balls in caves, or the best ball you have. The more Pokemon you catch, the higher rate you'll have for a "critical" capture, which instantly bags a creature.
Most of the time, I don't need to even damage Pokemon to catch them -- so don't think you need to beat every 'mon within an inch of their lives in every encounter. If I do need to beat on them though, what I usually do is employ the help of a "utility" Pokemon strictly for capturing. In my opinion, Pancham/Pangoro is one of the best creatures for the job, as it can learn Surf, Strength, and Cut. For the fourth move, give him False Swipe, which stops short of killing a Pokemon, leaving it at 1 HP. This way Pancham can not only get you anywhere you need to go map-wise for capturing, but it can also help you beat down hard to catch enemies -- it really helps that it's extremely strong in general.
Another go-to method for mass producing trading chips is training Eevees. As previously mentioned, look in yellow patches above Cyllage City and catch around 10 Eevees. Then follow the below methods for each type.