Yep. So those wiggling bushes have a chance to include special Pokemon. You might have maxed IVs or egg moves. Their levels might also be ten to fifteen levels above the standard for that area. Encountering more of a particular Pokemon increases the chance of a special encounter through the shaking grass/plains/ocean.
If you use that pokemon application I talked about earlier, and you've caught something, you can scan specifically for that Pokemon to appear in wiggling grass. Chaining together those encounters increases your chances of finding a shiny, much like chain fishing from XY.
Oh! Also, connect to wifi mystery gift, I think they're still running the shiny Beldum promotion. It knows Iron Head, Zen Headbutt, Hold Back (false swipe alternative), andsomething else I can't remember. And it's shiny, and it has the mega stone for Metagross!