Arceus is NOT available. And Master Balls are not meant for specific Pokémon...
Even with the 1%, they are pretty common.
Refer to the posts in this thread, DRG already mentioned this.
By quickly remembering... Kami Trio, Dogs, Weather trio, Celebi, JHMirachi, Deoxys, Mew, in a meaning Giratina and that's all I guess...
then how come you get a master ball saying you need it to catch arceus
u can't get kyurem