4 Options to obtain all starters:
- The easiest way to get the starters is to breed the one you started with and post it on the GTS in exchange for one of the other starters. Once you have it you can breed those new starters and have the starters in every evolution and your own region if you were traded something from another region for some reason.
- Restart the game, get a starter, push it to Pokemon bank. Then restart the game select a different starter, and again push to Pokemon bank, etc.
- If you don't have Pokemon bank trade with a friend for a random Pokemon that you don't mind losing. restart, choose different starter, etc.
- Find a friend that has all 3 starters and have him breed all 3 of them for you.
To get the remaining Pokemon you will need to learn how to breed. (I recommend getting a ditto for this).
All non-event legendaries are available in x, y, or, and as. You will need all 4 copies of the game or a friend that has the opposing copies to yours in or/as. OR Legendaries guide
You will need to find a way to get the legendary legendary bird that did not appear in your x/y copy. GTS will probably have something.
Mirrage island in or/as will get you most Pokemon not found naturally.
Event Pokemon your on your own for. Wait for an event or find a really good trade. In general you need an event Pokemon to get an event Pokemon.
You will need 2 3ds in order to get the trade with item Pokemon evolutions or you'll have to settle with whatever you find on the gts for this.
I the first 3 generations of Pokemon fully completed and many more but with lots of holes. I'm nearly 300hrs in. I have two ds's with every Pokemon game released for the ds/3ds system. (hg/ss d/p/p b/w b2/w2 x/y or/as) a full pokedex is a lot of work and will probably take a couple years to complete.