Triple Slash is the way to go. As soon as Rufus tries to approach Cloud, cast Triple Slash again to stop him. Rinse and repeat this for Rufus' second phase. Once you see Rufus casting any of his distance-based attacks, immediately cast Stop on him. At this point, you should have stagger-locked Darkstar and can quickly kill him. A perfect scenario of killing off Darkstar would be to have two full ATB bars. At the start of the third phase, immediately cast sleep on Rufus. After Rufus falls asleep, cast Braver on him, which should stagger him, and then use the punisher combo followed up by another Braver attack, and the fight's basically done.