Vivi can win on his own, with a little of Zidane's help. When you start, get back in the trolley and go to the Business District. Run to the second screen until you see Vivi being attacked by a Fang. Kill the Fang for him. After this, Vivi will start doing really well, beating out Freya about half of the time. After that, it's up to who happens to have the highest score when the time is up, usually it's Freya but if you help Vivi out he has a decent chance of winning.