Use it on any legendary that is hard to capture, like a Regi or the version exclusive.
The light ball you can get it by catching pikachu.first go and get a Pokemon who knows sweet scent and go to safari zone>after your there go to a spot where you last met pikachu and use sweet scent.96% it will come with oran berry or light ball
i think you have missed any events so the only way would be to use an action replay
Use it on Palkia.
weaken the Pokemon, give it a status problem, use a special pokeball on it if you can like if its a water type use a net ball, if its the opposite gender of your Pokemon use a love ball and so on, if you caught one already of the Pokemon use a repeat ball, confuse it use attract.
use a water type because if its fire it will burn him.if you use water use a great ball, master ball if you had one,and poke ball. if i were you use a great ball.
you can use your master ball
use a ultra/great ball
a master ball!!!
ultra ball
find one fight it and use an ultra ball or don't fight it and use a master ball
giratina
probaby kyogre