When Suicune uses doulbe team (clones itself) look to find the one with the shadow. The one with the shadow is the real one.$ Once all of the Suicunes' are gathered, use a pokemon assist that stops it, like your minuns/plusles assists. That's When you should capture it.
P.S. Once you see them all gathered, amd you use the assist, it may not work because by the time the pokemon uses it, they may have seperated. Also, if all of them have a shadow, or none of them do, just gather do the part with a $ in front of it. PLEASE RATE!!!!