I have seen a video on this and I didn't know if it could work. First, stand in a big area of grass with the roaming legendary in that route. Secondly, use pokeradar and go to the flashing patches. When you find a legendary, kill it and the chain starts and continue the chain. At the chain of 40, keep using the pokeradar until you find a shiny patch. It should be the shiny legendary you chained.
Pls tell me whether this will work. I'm in doubt. XD