I am not sure how many people know this, but if you play with a specific style, XY is actually challenging and one of the most fun experiences in the series.

How to do it:
Just fill up your dex and rotate the party every now and then. At the start have your starter and something like beedrill, dunsparce etc and depending on their lvl and strenght you should rotate them around when you arrive at Lumios city with other mons you have caught and the kanto starter.
Eventually you can bring them back and make them catch up with the share and use them for a route or gym. Then rinse and repeat. With this playstyle your pokemon always stay at their intended lvl and you get to experiment with new/old pokemon you have never used before.
With this method I was for example able to use an even lvl beedril on the fairy gym.
Other thing to note is that "empty" team slots should be for leveling up weak mons so they evolve.

I really suggest this out. It makes you like many mons you may not have used before and the Kalos dex is massive giving you a lot of choices.