I don't use Master Ball. Well, most of the time. d: When it comes capturing legendaries, I always try to use the regular type of Poké Balls—even on roaming ones. If I ever end up encountering a roaming legendary Pokémon, first thing I do is strike an attack to lower its health a bit. Then, I will go into the Pokémon Center and stack my team up with Pokémon that could help me capturing this legendary, such as those who know how to use Mean Look, Thunder Wave, False Swipe, etc. When I'm all set, I would start hunting this Pokémon. As soon as I encounter it, I'd use Mean Look instantly, so that it'd be blocked from escaping. After that, I'd have it paralyzed and bring it down to its lowest possible health. Now, it's ready for capturing! I could use any Poké Ball that I like (other than Master Ball, of course), and nothing would ever be able to stop me. (: Last time I've used a Master Ball was when playing Emerald, I think. I've used it once in Pokémon Y, but that doesn't totally count, since I reset the game immediately after the capture.