You could always use taunt like KrazyGlue said, but the problem with that is that the taunt will go away eventually, and you'll have to keep on laying it out. Another option, which I recommend, is to train a Pokemon that can learn both imprison and roar. And luckily for you, there is one!
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Vulpix learns roar at lvl 9 and then imprison at lvl 25. After you get what all moves you want for it, just evolve it and train it high enough so that it can both outrun Raikou and survive quite a few of its attacks, and you're all set.