First u have 2 have beat the elite four. after that go to the battle frontier and go to
the battle tower. clone a pokemon with the masterball if u already found it. if u dont kno
how 2 clone first deposit a pokemon u want to clone into any box. save yur game. withdraw
it and go to the multilink battle lady and talk to her. choose to challenge and choose 2
pokemon that r able to go into either 50 or open level. after she tells u, u need to save
say yes. yur game will freeze for about 1-2 seconds. after the game turns back to normal
turn of yur game. then if that pokemon was holding the masterball the clone will have it
2.  do this a lot of times. then find a rare or legendary and confront it. u dont have to
even fight it. just throw a masterball and it will catch it 100% of the time. thats how i
caught all the regis, kyogre, groudon, rayquaza, and latios.