Every pokemon in the Game can be captured with any pokeball, even a Giratina you can catch using a pokeball, the master ball is just a easy way to catch a pokemon ... read more
The Master Ball has a 100% catch rate for any Pokemon. There's no specific Pokemon that it's dedicated for, although many people prefer to use it against Legendaries like Giratina and Dialga, or save it for more elusive Pokemon that can flee battles, such as Cresselia. read more
No Pokemon is impossible to catch apart from the Master Ball. If you use Mean Look with a Crobat, Cressalia/Mesprit/Moltres/Zapdos/Articuno can't escape, and you can catch them with normal means. read more