Before you go to the sky piller go to the abanded ship on route 108 where u can get the luxery ball now when u fight raqwaza the lower his life the harder it is to catch him,so make sure his life is the coler yellow that way he will be on the 50-50 thing,any way when his life is in the yellow use a move to make him fall asleep then use the luxery ball and when that happens u better pray it works becuase there is a small chnce it still might not work

hope i helped ;)