There is no telling how to get shiny Pokemon.
Shiny Pokemon are very rare. If you are asking how that person got so many shinies, He probably used cheats. If you want so many shinies, why not buy an Action Replay? But if you don't want to use a cheat code, that's fine. Keep searching for shinies.
Another way to obtain shiny Pokemon?
Trade! Just ask for the person that has alot of shiny Pokemon his/her friend code and you can trade! I don't know if this is a good idea, but try if you want.
How to get shiny legendaries?
This is veeeery simple although you need Action Replay. Find a cheat like "All wild Pokemon shiny", "Shiny legendaries" or "Shiny (legendary's name) in Box (number)". Then, play your game, (press buttons in order if needed) and check if the cheat worked. If you don't know the difference about "Shiny legendaries" and "Shiny - in Box -", here's the difference. Shiny legendaries are for if you have masterballs. You can capture them easily. But if you don't have them and have a hard time capturing, use the Shiny () in Box (). It will be automatically sended to your box (without capturing it).
But here's another way to get shiny legendaries and this is simple. Save on front of a legendary. Interact with it. If it isn't shiny, turn of the power then play again. Repeat this activity until you get a shiny. You can use the other "how to get shiny legendaries" if you want.
Sorry if there are a lot of cheats here, but hope this helps.