If I remember right X and Y have the horde mechanic so you can abuse that, I did that for a few shinies.
Also, I could be wrong about this, but I THINK that series also has an area under a bridge that has a 100% feebas spawn rate - I used that to farm a shiny feebas. It’s been a very long time since then, so it could be a different generation.
Good lucky shiny hunting!
Edit: just did some research and it seems the Feebas spawn is on ORAS. So it seems the horde mechanic is the best way to cheese out some shinies the fastest.