The first thing that you should do is get your ditto. In X/Y the best way to get a good IV Ditto is through the friend safari, however you need to have a friend whose friend code creates a normal type friend safari, and you also need to get lucky in having a Ditto be in that friend safari. It is slightly easier in ORAS as you can use the DexNav on the Mirage Spot that contains Ditto to have a higher chance of getting a Ditto with good IVs.
Bulbapedia Guide for Mirage Spots: https://bulbapedia.bulbagarden.net/wiki/Mirage_spot
To answer the main question, the best way to get Pokemon with good IVs in either game is to breed. In my opinion the best method of breeding is to first get a Pokemon with Flame Body to be in the front of your party. The ability Flame Body halves the steps it takes to hatch an egg. I prefer to use Talonflame as it can also use Fly, which is very helpful. The second thing you need to do is get the Pokemon you want to breed, and your Ditto. If either of these Pokemon have a Nature that you'd like to keep on the offspring, give that Pokemon the Everstone, which gives the Pokemon holding it a 100% chance to pass down its nature. Next give the other Pokemon (the one not holding the everstone,) a Destiny Knot, which enables the parents to pass down 5 IVs, randomly chosen between the two parents, to the offspring. Put both of these Pokemon in the Daycare and you're good to go!
You can use the infinite running/biking loop at the Battle Resort in ORAS, or the one in Lumiose City for X/Y to get your eggs and hatch them! Once you get a baby Pokemon with better IVs than the parent Pokemon, you can swap the parent Pokemon out in favor of the better baby Pokemon. With a little bit of patience (a LOT of patience*) you will eventually get a Pokemon with the IVs that you want!
